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PRINCIPLES OF INSECT IMORPHOLOGY 



McGraw-Hill Publications in the Zoological Sciences 
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^PRINCIPLES OF 

INSECT MOEPHOLOGY 


R. E. SNODGRASS 


United Slates Dcparlmcnl of Agriculture 


Bureau of Entomology and Plant Quarantine 


McGRAW-HILL BOOK COMPANY, Inc. 


NEW YORK AND LONDON 

1935 


COPTHIGHT, 1935, BY THE 
McGhaw-Hilij Book Company, Inc. 

PRINTED IN THE HOTTED STATES OP AMERICA 

All rights reserved. This book, o, 

'parts thereof, may not be reproduced 
in any form vnthout permission of 
the vvblishers. 

xiii 


59510 



PREFACE 


'J'he principal value of facts is that they give us something to think 
about. A scientific textbook, therefore, should contain a fair amount of 
reliable information, though it may be a matter of choice with the author 
whether he leaves it to the reader to formulate his own ideas as to the 
meaning of the facts, or whether he attempts to guide the reader’s 
thoughts along what seem to liim to be the proper channels. The 
writer of the present text, being convinced that generalizations ai’e more 
important than mere knowledge of facts, and being also somewhat 
partial to his own way of thinking about insects, has not been able to 
refrain entirely from presenting the facts of insect anatomy in a way to 
suggest relations between them that possibly exist only in his own mind. 
Each of the several chapters of this book, in other words, is an attempt 
to give a coherent morphological view of the fundamental nature and the 
apparent evolution of a particular group of organs or associated struc- 
tures. It is more than likely, practically certain, that many of the 
generalizations here offered will soon be modified or superseded by other 
generalizations, but they will have served their purpose if they induce 
critical students to make a wider and more thorough study of the prob- 
lems of insect morphology. 

Two notable books have appeared recently in entomology: one, 
“Lehrbucb der Entomologie,” by Dr. Hermann Weber of Danzig, in its 
first edition; the other, “A General Textbook of Entomology,” by Dr. 
A. D. Imms of Cambridge, England, in its third edition. In preparing 
the present text the writer has made a special effort to concur with the 
authors of these books in the matter of anatomical terms, in order that 
students may as far as possible be spared confusion in turning from one 
treatise to another. Unfortunately, however, there is still much unavoid- 
able discrepancy in the use and application of anatomical names in 
entomology. The troi^e, ,ii^ Jgrge measure, can be blamed on the 
insects themselves, since they will not entirely conform with any plan 
of nomenclature or with any scheme we can devise for naming their 
parts consistently. To make clear the meaning of terms as used in this 
text, therefore, a glossary of definitions is appended to each chapter, 
wherein, also, will be found the German equivalents of many of our 
English and Latinized technical names. 

In making acknowledgments, the writer must first of all declare his 
indebtedness to the Bureau of Entomology, United States Department 



vi 


PREFACE 


of Agriculture, for the experience and information acquired in the course 
of his many years of official service. Most of the illustrations accom- 
panying the text that are not accredited to particular sources are the 
property of the Bureau of Entomology and Plant Quarantine, and 
many of them have been published in the Miscellaneous Collections and 
the Annual Reports of the Smithsonian Institution of Washington, D.C. 
For the use of these figures the writer hereby expresses his thanks both 
to the Bureau and to the Smithsonion Institution. With regard to 
illustrations borrowed from other works, the writer is particularly 
indebted to Professor Hermann Weber, of Danzig, for permission 
to use figures from his “Biologie der Hemipteren” and “Lehrbuch 
der Entomologie.” The rest of the illustrations, each accredited to its 
proper source in the scientific iournals, have been freely drawn from the 
common heritage of entomology contributed by the many workers in 
many lands who have devoted themselves to the study of insects. To 
my wife, Ruth H. Snodgrass, credit is due for the typing of the manu- 
script and for much of the work of indexing and proofreading. 

R. E. Snodgrass. 

Washington, D. C. 

May, 1935. 



CONTENTS 

i’RBlACE V 

CHAPTER I 

Introduction 1 

CHAPTER II 

General Organization and Development 14 

Glossary of Embryological Terms 44 

CHAPTER III 

The Body Wall and Its Derivatives 48 

1. The Body Wall 48 

2. External Processes of the Body Wall 55 

3. Sense Organs 59 

4. Ectodermal Glands, Corpora Allata, Oenocytes 60 

5. Muscle Attachments on the Body Wall 62 

6. Moulting 64 

Glossary of Terms AppUed to the Body Wall 68 

CHAPTER IV 

Body Regions, Sclerites, and Segmentation 70 

Glossary of General Terms Applied to the Body Segments and the Skeletal 
Plates 81 

CHAPTER V 

The Segmental Appendages of Arthropods 83 

Glossary of Terms Applied to the Appendages 98 

CHAPTER VI 

The Head 100 

1. General Morphology of the Arthropod Head 100 

2. Structure of the Definitive Insect Head 104 

3. Special Modifications in the Structure of the Head 118 

Glossary of Terms Applied to the Head 127 

CHAPTER VII 

The Head Appendages 130 

1. Preantennal Appendages 130 

2. The Antennae 131 

3. The Postantennal Appendages 133 

4. The Mandibles 133 

5. The Superlinguae 139 

6. The Maxillae 141 

7. The Labium 145 

8. Glands of the Head Appendages 153 

Glossary of Terms Applied to the Head Appendages 155 

vii 



CONTENTS 


viii 


CHAPTER VIII Pj,qb 

The Thohax 157 

1. Evolution of the Thorax 157 

2. The Neck 159 

3. General Structure of the Thorax 160 

4. The Prothorax 172 

5. The Pterothorax 173 

6. The Thoracic Muscles 186 

Glossary of Terms Applied to the Thorax 190 

CHAPTER IX 

The Thoracic Legs 193 

1. The Structure of the Legs 193 

2. Muscles and Mechanism of the Legs 200 

Glossary of Terms Applied to the Parts of an Insect’s Leg 209 

CHAPTER X 

The Wings 211 

1. Origin and Evolution of the Wings 211 

2. Development of the Wings 214 

3. Structure of the Wings 215 

4. The Wing Muscles 228 

5. The Wing Movements 233 

6. Insect Flight 240 

Glossary of Terms Applied to the Wings 243 

CHAPTER XI 

The Abdomen 246 

1. General Structure of the Abdominal Segments 247 

2. The Abdominal Musculature 257 

3. The Abdominal Appendages 267 

CHAPTER XII 

The Organs of Ingestion 280 

1. The Preoral Cavity 281 

2. The Cephahc Stomadaeum 284 

3. The Feeding Mechanism of Neuroptera and Coleoptera 286 

4. The Feeding Mechanism of Hymenoptera 295 

5. The Feeding Mechanism of Lepidoptera 302 

6. The Feeding Mechanism of Diptera 311 

7. The Mouth Parts of Siphonaptera 325 

8. The Feeding Mechanism of Thysanoptera 326 

9. The Feeding Mechanism of Hemiptera 328 

10. The Feeding Mechanism of Anoplura 344 

CHAPTER XIII 

The Alimentary Canal 347 

1. Development of the Alimentary Canal 347 

2. General Structure of the Alimentary Canal 348 

3. The Stomodaeum 349 

4. The Mesenteron 359 

5. The Proctodaeum . 374 



CONTENTS 


ix 

Page 

6. The Filter Chamber 383 

Glossary of Terms Applied to the Alimentary Canal 387 

CHAPTER XIV 

The Organs of Distribution, Conservation, and Elijhnation 389 

1. The Blood 389 

2. The Organs of Circulation 397 

3. The Fat-body 407 

4. The Ocnocytcs 410 

5. The Corpora allata 411 

6. The Organs of Elimination 413 

Glossary of Terms Used in This Chapter 420 

CHAPTER XV 

The Respiratory System 422 

1. The Integument as a Respiratory Organ 423 

2. Blood Gills 424 

3. The Tracheal System 425 

4. General jMechanism of Tracheal Respiration 452 

Glossary of Terms Applied to the Respiratory System 461 

CHAPTER XVI 

The Nervous System 464 

1. General Structure, Organization, and Function of the Nervous System. . 465 

2. The Central Nen'ous System 472 

3. The Stomodacal Ners’ous System 501 

4. The Peripheral Nervous System 503 

Glossary of Terms Applied to the Ncia'ous System 507 

CHAPTER XVII 

The Sense Organs 510 

1. General Structure and Classification of Insect Sense Organs 512 

2. The Hair Organs 515 

3. The Campaniform Organs 521 

4. The Plate Organs 523 

5. The Scolopophorous Organs 525 

6. The Eyes 528 

Glossary of Terms Applied to the Sense Organs 548 

CHAPTER XVIII 

The Internal Organs of Reproduction 550 

1. The Female Organs 552 

2. The Male Organs 567 

3. General Morphology of the Reproductive Organs 573 

Glossary of Terms Applied to the Internal Reproductive Organs 578 

CHAPTER XIX 

The Organs of Copulation and Oviposition 581 

1. The Male Genitalia 582 

2. The Female Genitalia 607 

Glossary of Terms Applied to the External Genitalia 620 

References 625 

Index 647 




PRINCIPLES OF INSECT 
MORPHOLOGY 


CHAPTER I 
INTRODUCTION 

Morphology, in the biological sense, is the science of form in living 
organisms. Anatomy is the determination of structural facts. Morphol- 
ogy seeks to find the reason for structure, and to understand the relation 
of different structural forms to one another. Morphology, therefore, 
must be intimate with function, since it must see forms as plastic physical 
adaptations to the work to be performed. A few physiological functions 
are basic to all organisms ; they are essential to the continuance of matter 
in a lining state. The v^arious structural types of organisms are special 
ways of accomplishing these functions, that is, for doing the same things 
in different w'ays or under different circumstances. Some represent 
improvements in the machinery along established lines ; others represent 
changes or new ideas developed along new and divergent lines. The 
morphologist, therefore, though primarily a comparative anatomist, in 
order properly to develop his subject, must give attention to the worldng 
of the physical mechanisms with which he deals in his anatomical studies, 
he must look for the significance of structural modifications and innova- 
tions, and he must understand the basic physiological functions that 
underlie organic form. 

In the study of insect morphology we cannot confine ourselves to the 
limits of entomology. The fundamental organization of insects was 
established long before insects became a specialized group wdthin the 
phylum of the Arthropoda, and the basic structure of the arthropods is 
much older than the arthropods themselves. As organisms evolve, 
important structures are often so modified that their true nature becomes 
obscured; but the same structure is not likely to be modified to the same 
degree in all related groups, or in all members of the same group. Struc- 
tural modification has been carried to a high degree in all the arthropods, 
affecting some organs in one group, others in another group ; and particu- 
larly is this true of the insects. Hence, in the discussion of the mor- 
phology of insect organs given in the following chapters, many references 

1 



2 


PRINCIPLES OF INSECT MORPHOLOGY 


will be made to corresponding parts in other arthropods, while, for an 
understanding of the more fundamental structures of the arthropods, it 
will be found necessary to go back to the segmented worms and to those 
wormlike creatures known as onychophorans. Before taking up the 
particular subject in hand, therefore, we must know something of the 
distinctive structural features of the Annelida the Onychophora, and 
the major groups of the Arthropoda. 

THE ANNELIDA 

The typical annelid worms are elongate cylindrical animals divided 
transversely into a series of segments. The mouth is situated ventrally 
between the first segment and a preoral lobe (prostomium) ; the anus is 
terminal in the last segment (periproct). The segments between the 
prostomium and the periproct are true metameres, or somites, produced 
by segmentation of the primitive body region anterior to the periproct. 
Some of the annelids are provided with lateral segmental appendages 
(parapodia), which are hollow evaginations of the body wall, movable 
by muscles inserted on or within their bases. 

The body cavity of the Annelida is the coelome. It is often divided 
transversely by intersegmental septa into segmental coelomic cavities. 
The alimentary canal is a tube extending through the body from the 
mouth, situated ventrally between the prostomium and the first somite, 
to the anus, which is terminal on the periproct. A blood vascular sys- 
tem is well developed in some forms by enclosure of tracts of the haemo- 
coele in mesodermal walls. Usually there is a median dorsal vessel and a 
median ventral vessel connected by lateral trunks, from which are given 
off branches to the various organs of the body. The excretory system 
consists of paired segmental tubes (nephridia) opening, on the one hand, 
into the coelome, and, on the other, to the exterior. Respiration takes 
place either directly through the body wall or by means of gills, which are 
evaginations of the integument. 

The annelid nervous system includes a median prostomial ganglion, 
the brain (archicerebrum), lying dorsal to the alimentary canal, and a 
ventral nerve cord consisting of double segmental ganglia united by paired 
connectives. The brain and the first ventral ganglia are united by 
connectives embracing the oesophagus. The brain innervates whatever 
sense organs, such as palpi, tentacles, and eyes, may be located on the 
prostomium; it is often differentiated into a forebrain (protocerebrum) 
and a hindbrain (deutocerebrum). The forebrain may contain well- 
developed association centers in the form of stalked bodies, or corpora 
pedunculata. 

The germ cells of the mature annelid occur in groups imbedded in the 
mesodermal lining of the coelome, the simple organs thus formed being 



INTRODUCTION 


3 


(he gonads (ovaries and testes). The ripening ova and spermatozoa are 
discharged from the gonads either into the general coelomic cavity, from 
Avliich they escape through tlie nephridia or through pores of the body 
wall, or into siiccial coelomic receptacles connected bj’’ ducts with the. 
exterior. The young annelid larva has a characteristic form and is 
known as a trochophore. 


THE ONYCHOPHORA 

The onyehophorans, including Pcnpalus and related genera, are 
wormlike animals resembling the annelids in many respects. Though 
.‘segmentation is not evident in the C3'lindrical bodj’’ or in the somatic 
musculature, the presence of a scries of paired lateroventral ambulatoi'y 
appendages gives the animal a segmented appearance. The “legs” 
resemble the annelid parapodia in that each is .a hollow evagination of the 
bodj’ wall movable bj’’ four sets of muscles reflected into the appendage 
from the somatic wall. The mouth is situated anteriorly on the ventral 
surface at the base of a prostomial lobe. The prostomium bears a pair of 
tentacles and a pair of simiile ej’es. An extraoral mouth c.avity contains 
the true oral opening and a pair of stronglj’- muscul.atcd mouth hooks. 

The bodj' cavitj* of the Onj'chophora is continuous through the length 
of the animal. The circulatoiy .sj'stem consists of a dorsal vessel only, 
which has paired openings into the bodj' cavity between each pair of legs. 
The excretorj’ organs arc ncjihridia similar to those of Ajinclida, opening 
cxtcrnallj’ on the bases of the legs. Delicate internal air tubes (tracheae), 
arising in grouixs from pits scattered irrcgularlj' over the integument, 
probablj' subserve respiration. The nervous sj'stem consists of a dorsal 
brain located in the head, and of two long lateral nerve cords in which 
ganglia are but little difTcrentiated. The brain inncrv.ates the tentacles, 
the cj'cs, and the mouth hooks. The reproductive organs in each sex 
are a pair of long tubular sacs, the ducts of which unite in a median e.xit 
tube that oijcns vcntrallj' near the posterior end of the body. Most 
species of Onychophora arc viviparous, the embryo being developed 
within the oviducts of the female. The young animal takes on directly 
the form of the adult. 

The Onj'chophora h.ave often been regarded as primitive arthropods, 
but there is little in their organization that conforms with arthropod 
structure. Their relationships are undoubtedly with the Annelida, but 
the fact that the young .at no stage have any resemblance to a trochophore 
larva would seem to indicate that the Onychophora are not derived from 
typical annelids. The Onychophora, in fact, have .an ancient lineage of 
their own; fossil forms arc known from the Middle Cambrian that closely 
resemble modern species, except for the smaller number of legs. 



4 


PRINCIPLES OF INSECT MORPHOLOGY 


THE ARTHROPODA 

The arthropods have an annulate body and segmental appendages. 
Their distinctive features are the jointing of the appendages and a group- 
ing of the body segments, each appendage being composed of a number of 
lim b segments (podites) individually movable by muscles, while the 
body segments are segregated to form more or less distinct trunk sections 
(tagmata). The integument is usually hardened by the deposition of 
sclerotizing substances in definite areas of the cuticula, forming exo- 
skeletal plates (sclerites) to which most of the muscles are attached. 
The intervening membranous areas allow of movement between the 
plates. This character has given the arthropods an unlimited field for 
the development and evolution of exoskeletal mechanisms both in the 
trunk and in the appendages. 

The composition and specialization of the trunk sections, or tagmata, 
are characteristic of each of the several major groups of arthropods. The 
most constant and distinctive tagma is the head. In its simplest form 
the definitive head represents the embryonic protocephalon, consisting of 
a large preoral region and usually the first postoral somite. Generally, 
however, it includes a gnathal region (gnathocephalon) formed of at least 
three succeeding somites. The body region following the head may 
preserve a uniform segmentation and simple structure, or it may be 
variously differentiated into a thorax and an abdomen. In some forms 
the cephalic region and a varying number of succeeding somites are 
combined in a “cephalo thorax,” or prosoma, distinct from the abdomen. 
The primitive terminal segment (telson) is probably not a true somite, 
but an endpiece of the body bearing the anus, corresponding to the peri- 
proct of the Annelida. 

The appendages of the trunk include a pair of procephalic antennae 
(antermules), and a double series of segmented, postoral, ventrolateral 
limbs, potentially a pair on each segment but the last. The postoral 
appendages become variously modified in adaptation to functional special- 
izations. A typical arthropod limb consists of a basis (coxopodite) 
movable anteroposteriorly on the body, and of a six-segmented shaft 
(telopodite) movable in a vertical plane on the basis. Endite and exite 
lobes of the limb segments are frequently developed into specialized 
appendicular processes. 

The definitive alimentary canal includes long anterior and posterior 
sections (stomodaeum and proctodaeum) derived from the ectoderm. 
Typical segmental nephridia are absent. The blood vascular system 
is variously developed, but in plan it conforms with that of the Annelida. 
Respiration takes place either through the general body integument or by 
means of evaginations (gills) or invaginations (tracheae) of the body wall. 



INTRODUCTION 


5 


The nervous system of the arthropods has the same general structure 
as that of the Annelida. The primitive brain consists of a preoral body 
of nerve tissue lying above the stomodaeum, which is differentiated into 
protocerebral (ocular) and deutocerebral (antennular) regions, except in 
forms lacking antennae. The definitive brain in most groups, however, 
is a syncerebrum, since it includes also the first pair of ganglia of the 
ventral nerve cord, which secondarily become tritocerebral brain lobes. 
The protocerebrum contains often highly developed corpora pedunculata 
and the ocular centers; the deutocerebrum innervates the antennules 
(first antennae) ; and the tritocerebrum the first pair of postoral append- 
ages (chelicerae or second antennae). 

The visual organs of the arthropods include dorsal (median) and 
lateral eyes innervated from the protocerebrum. The dorsal eyes are 
always simple ocelli, located usually on the upper or anterior surface of the 
head, but in Xiphosura, and possibly in the trdobites and eurypterids, 
there is a pair of rudimentary ventral eyes on the deflected under surface 
of the head before the mouth. The number of dorsal eyes varies from one 
to eight, but often none is present; primitively there were perhaps two 
pairs; a single median dorsal ocellus probably represents the ocelli of one 
pair united. The lateral eyes are typically compound, beiug formed of 
groups of simple optic units composing a single organ, but often they are 
represented by groups of distinct ocelli. 

The reproductive organs are mesodermal sacs enclosing the germ 
cells. The paired mesodermal exit ducts open either separately to the 
exterior or into a common median outlet tube of ectodermal origin. The 
position of the genital aperture is variable. 

The Arthropoda include three major groups, namely, the Trilobita, 
the Chelicerata, and the Mandibulata. 

The Trilobita 

The trilobites are extinct creatures that flourished throughout the 
Paleozoic era but were most abundant during the Cambrian and Ordo- 
vician periods. They are the most generalized of known arthropods. 
The body is usually oval and flattened and carries ventrally a double 
series of jointed limbs. The trunk is divided into a head and two body 
regions known as the thorax and the pygidium, but the name "trilobite” 
is derived from the apparent triple division of the trunk lengthwise 
into an elevated median area (the axis, or rhachis) and two depressed 
lateral areas (pleurae). The head, which appears to include the 
prostomium and four somites, is covered by a dorsal carapace; the 
thorax consists of a variable number of free segments; the pygidium con- 
tains several segments, which, except in certain earlier forms, are united 
in a caudal shield. Each body segment, except the last, bears ventrally 



6 


PRINCIPLES OF INSECT MORPHOLOGY 


a pair of jointed appendages. On the upper surface of the head in most 
species are a pair of compound lateral eyes, and in some forms a median 
tubercle which appears to be a simple dorsal eye; on the under surface 
is a pair of small spots which some writers believe to be ventral eyes. 

The distinctively generalized feature of the trilobites, as compared 
with the other arthropods, is the lack of specialization and structural 
differentiation in the segmental appendages. The first pair of appendages, 
which probably are procephalic antennules, are filamentous and multiar- 
ticulate. The rest are without doubt postoral limbs. They are practi- 
cally all alike except that some of the more anterior ones may have a 
greater number of segments than the others. The trilobite limb pre- 
serves the typical form and fundamental^ structure of all arthropod 
appendages. The basis supports a large exite (epipodite) bearing a 
series of thin, closely set plates or filaments, which probably functioned 
as gills. The telopodite is usually six segmented. 

The trilobites appear to be related, on the one hand, to the Xiphosura, 
and, on the other, to the phyllopod crustaceans, since they have features 
characteristic of both these groups. They are not literally the ancestors 
of the other arthropods, however, since along with the trilobites there 
lived the highly specialized eurypterids and a large and varied crustacean 
fauna, but the trilobites are probably more closely related to the ancestral 
arthropods than are any other known forms. 

The Chelicerata 

In both the chelicerate and the mandibulate arthropods the seg- 
mental appendages are diversified in form and function, and some of them 
are suppressed. The most generally distinctive features that separate the 
Chelicerata from the Mandibulata are the suppression of the antennules, 
and the modification of the first postoral appendages to form a pair of 
chelicerae, which typically are pincerlike feeding organs. 

The body segments are grouped into two trunk regions, a prosoma and 
an abdomen. The first includes the protocephalon and the first six 
postoral somites, which are always more or less united. The abdomen 
varies in length and may be distinctly segmented, though in the higher 
forms it is usually short and its segmentation indistinct or suppressed. 
The prosoma bears six pairs of limbs, including the chelicerae, all of which, 
except the chelicerae, are generally leglike in form. The telopodites of 
some of the appendages often contain seven segments instead of six by 
the interpolation of a "patella” between the femur (meropodite) and the 
tibia (carpopodite). Abdominal appendages are usually absent, but in 
the more generalized forms they are retained in modified shape and 
may have gill-bearing epipodites as in the Trilobita. 



INTRODUCTION 


7 


The brain of the Chelicerata is a syncerebrum composed of the primi- 
tive cerebrum and the ganglia of the cheliceral segment, but, owing to 
the loss of the procephalic antennules, the deutocerebral centers are 
suppressed, and the brain consists of the protocerebrum, which innervates 
the eyes, and of the tritocerebrum, which innervates the chelicerae. 
The ocular organs include median dorsal eyes, lateral eyes, and in some 
cases ventral eyes. The lateral eyes are compound in more primitive 
forms; in others they are represented by groups of simple eyes. 

The Chelicerata include the Eurypierida, the Xiphosura, the Pycno- 
gonida, and the Arachnzda, 

Eurypterida. — ^The eurypterids are extinct Paleozoic arthropods 
that lived from the Cambrian to the Carboniferous period but attained 
their greatest development in the Silurian and Devonian. They were 
aquatic, mostly fresh-water, or mud-inhabiting creatures. While the 
majority were relatively small, less than a foot in length, some became 
the largest of all known arthropods, reaching a length of (5 or 7 feet. 
The segments of the prosoma are united; the abdomen consists of 12 free 
segments, the last bearing a telson, which is usually a long tapering 
spine but is plate-like in some forms. The chelicerae are long or short. 
The next four pairs of limbs are generally small, and the sixth pair long 
and flat, evidently swimming organs ; but in some species the legs are all 
long and slender. The first five segments of the abdomen have plate-Jike 
appendages overlapping each other and concealing gills. On the upper 
surface of the cephalic region are a pair of compound lateral eyes and two 
small simple dorsal eyes; on the under surface, according to Stormer 
(1934), there appears to be in some forms a pair of ventral eyes. 

Xiphosura. — The modern members of this group, commonly known as 
horseshoe crabs or king crabs, have so many points of resemblance 
with the ancient eurypterids that the two groups are often classed together 
as the Merostomata, and certain exi^met forms make the connection even 
closer. The body of the horseshoe crab is distinctly divided between the 
prosoma and the abdomen; in each, part the segments are united and 
covered by a large dorsal carapace, the second ending in a long spine-like 
telson. The six thoracic appendages are all chelate in the female. The 
abdomen contains only six segments, which bear plate-like appendages 
veiy similar to those of the anterior part of the abdomen of tlie euryp- 
terids, each except the first having a large gill-bearing epipodite. On 
the head are a pair of compound lateral eyes, a pair of simple median 
dorsal eyes, and a pair of rudimentary median ventral eyes. 

Pycnogonida, or Pantapoda. — The pycnogonids are aberrant arthro- 
pods commonly kiiown as sea spiders. From the nature of their append- 
ages they appear to belong to the Chelicerata. 



8 


PRINCIPLES OF INSECT MORPHOLOGY 


Arachnida. — Here are included the scorpions, the solpugids, the 
phalangids, the spiders, the ticks, and the mites. In most forms the 
trunk is divided in the typical chelicerate manner into a prosoma and an 
abdomen, but in the solpugids the prosoma is constricted between the 
fourth and fifth pairs of limbs. The abdomen may be long and distinctly 
segmented, in which case the distal part is narrowed and has the form of a 
jointed “tail,” but in the spiders and mites the abdomen is short and 
rotund, with indistinct or suppressed segmentation. The prosomal 
appendages include the chelicerae, a pair of pedipalps, and four pairs of 
legs. Abdominal appendages are either absent or represented by modi- 
fied rudimentary structures. The cephalic region of the prosoma 
generally bears a group of simple dorsal eyes, and, in some forms, on each 
side one to five simple lateral eyes ; the lateral eyes are never compound. 
Paleontologically the oldest known arachnids are contemporaneous with 
the trilobites and eurypterids. 

The Mandibulata 

The mandibulate arthropods differ from the Chelicerata in two 
characteristic features, namely, (1) the retention of the procephalic 
antennules, and (2) the modification of the bases of the second postoral 
appendages to form a pair of biting, jawlike feeding organs, the mandibles. 
The appendages of the first postoral segment, corresponding to the 
chelicerae of the Chelicerata, are present in most Crustacea as a pair of 
large biramous antennae (second antennae), but in the other mandibulate 
groups they are suppressed or are represented only by embryonic rudi 
ments. The telopodites of the mandibles may be retained in Crustacea in 
the form of “palpi,” but otherwise they are lost. The first two post- 
mandibular appendages are generally modified as accessory feeding 
organs (the first and second maxillae), though in some forms they are 
reduced and more or less rudimentary. The following appendages may 
all have a uniform structure, or they may be variously modified on differ- 
ent regions of the body; but a patellar segment is never present in any of 
them. 

Tagmosis is variable in the Mandibulata. The head may consist 
of the proto cephalon only (including the second antennal somite), but 
generally it contains a gnathocephalic section composed of the mandibular 
and the two maxillary somites, to which may be added the appendages, 
at least, of the next trunk segment. In the malacostracan Crustacea 
the protocephalon, the gnathal segments, and a varying number of 
succeeding segments are more or less combined in a “cephalothorax.” 
The Myriapoda and Hexapoda have a distinct head including four post- 
oral somites; the body of the first group shows no tagmosis, but in the 
hexapods it is differentiated into a thorax and an abdomen. 



INTRODUCTION 


9 


The brain of the Mandibulata is well differentiated into a proto- 
cerebrum and a deutocerebrum, innervating respectively the eyes and 
the antennules (first antennae) ; generally it includes also the first postoral 
ganglia of the ventral nerve cord, innervating the second antennae when 
these appendages are present; these ganglia become the tritocerebral 
lobes of the definitive brain, though they retain their primitive ventral 
commissure. The mandibular and both maxillary pairs of ganglia are 
united in a composite suboesophageal ganglion contained in the head when 
the latter includes the gnathal segments. A stomodaeal nervous system 
is usually present, having its principal connections with the tritocerebral 
ganglia. 

The major groups of the Mandibulata are the Crustacea, the 
Myriapoda, and the Hexapoda. 

Crustacea. — ^The crustaceans include the phyllopods, the barnacles, 
the shrimps, crayfish and lobsters, the crabs, and related forms. The 
Crustacea are distinguished from all the other arthropods by a biramous 
structure of the limbs, each appendage typically having an outer branch 
(exopodite), arising from the basal segment (basipodite) of the telopodite, 
and an inner branch (endopodite), which is the shaft of the telopodite 
distal to the basipodite. The coxopodite, in many forms, supports a gill- 
bearing epipodite, which apparently corresponds to the similar basal 
appendicular organ of the limbs of Trilobita and the abdominal append- 
ages of Xiphosura. 

Tagmosis is variable in the Crustacea. The proto cephalon, including 
the tritocerebral somite, may constitute a small primitive head distinct 
from the rest of the trunk, or it may be united with several somites 
following to form a composite head; or, again, a variable number of 
segments in the thoracic region are more or less united with the head in a 
cephalothorax distinct from an abdomen. 

The first antennal appendages (antennules) are usually filamentous 
and multiarticulate; they are never biramous. The tritocerebral 
appendages (second antennae) are typically biramous and thus show that 
they belong to the series of postoral limbs, though the body segment 
bearing them forms a part of the protocephalon. The mandibles are 
always well developed; the two pairs of maxillae are small, sometimes 
rudimentary. The next three pairs of appendages are termed maxillipeds : 
the following five are differentiated in higher forms primarily as ambula- 
tory organs (periopods). The abdominal appendages (pleopods) are 
usually different from the thoracic appendages and are often specially 
modified. Most of the appendages may be modified for purposes of 
swimming. 

The brain in some of the lower Crustacea is a primitive cerebrum 
differentiated into a protocerebrum and deutocerebrum containing respec- 



10 


PRINCIPLES OF INSECT MORPHOLOGY 


lively the ocular and first antennal centers. In most forms, however, the 
definitive brain is a syncerebrum including the second antennal ganglia 
as tritocerebral lobes. The Crustacea have compound lateral eyes similar 
in structure to those of insects. 

Myriapoda. — The common myriapods are easily distinguished 
from the other terrestrial arthropods by their slender forms and many 
legs, but a more generally distinctive character is the division of the trunk 
into only two parts, a head and a body. The head bears a pair of preoral 
antennae (antennules) and probably includes not more than four postoral 
somites. The appendages of the first somite are absent, those of the 
second are the mandibles, the other two pairs are variable in structure. 
The body is typically long and uniformly segmented, though some of the 
segments may be reduced, and in two groups the segments are united in 
pairs. Most of the primitive segments bear each a pair of legs. The 
myriapods are terrestrial animals; the majority of them have tracheal 
invaginations of the body wall for respiration. Eyes, when present in 
modern forms, consist of groups of simple lateral eyes, which approach 
the compound type in Scutigera. Certain Permian diplopods are said to 
have had large compound eyes. 

The Myriapoda include two principal groups. In the members of 
one group (progoneate forms) the reproductive organs open near the 
anterior end of the body; in those of the other group (opisthogoneate 
forms) the genital opening is at the posterior end of the body. The 
progoneate myriapods include the Diplopoda, the Pauropoda, and the 
Symphyla; the opisthogoneate group consists of the Chilopoda 
(centipedes). In the diplopods and pauropods most of the body somites 
appear to be united in successive pairs, at least dorsally; the legs of the 
diplopods occur in pairs on each double segment. The pauropods have 
branched antennae. 

Hexapoda. — The hexapods are well characterized by the feature of 
their organization from which they get their name, which is the invariable 
specialization of three pairs of appendages as legs. The legs are always 
the appendages of the first three postgnathal somites ; the latter constitute 
a definite locomotor center, the thorax, distinct from the abdomen, which 
seldom bears organs of locomotion. The head has apparently the same 
composition as in the Myriapoda, since it always includes three pairs of 
gnathal somites. The abdomen never has more than 12 segments 
(11 true somites), of which the last is the periproct. 

The appendages of the head include a pair of pro cephalic antennae 
(antennules), embryonic rudiments of second antennae, a pair of man- 
dibles, and two pairs of maxillae. The second maxillae are united in a 
median composite organ known as the labium. The thoracic appendages 
are the three pairs of legs. Abdominal appendages may be present on 



INTRODUCTION 


11 


any of the 11 true somites of the abdomen, but they are always greatly 
modified or rudimentary and when present assume a variety of forms; 
generally most of them are absent in the adult stage. The appendages 
are never biramous in a manner comparable with the appendages of 
Crustacea, but epipodites may be present on the coxopodites. 

The brain of the hexapods has distinct protocerebral, deutocerebral, 
and trito cerebral centers, the last formed of the ganglia of the second 
antennal somite. The lateral eyes, when present, are typically com- 
pound, but in some adults and in many larval forms they are replaced by 
groups of simple eyes. Three dorsal ocelli are commonly present, the 
unpaired ocellus being median, anterior (or ventral) to the others, and 
probably double in its origin. 

The genital ducts open either separately or through a median tube of 
ectodermal origin. The paired duets of the female may open between 
the eleventh and twelfth segments of the abdomen (Protura), or between 
the seventh and eighth (Ephemerida). The median oviduct opens on the 
seventh, eighth, or ninth abdominal segment, the median ejaculatory 
duct on the ninth segment, except in Collembola where the genital open- 
ing in each sex is between the fifth and sixth definitive segments of the 
abdomen. 

The Hexapoda include several more or less distinct groups, which are 
the Protura, the Collembola, the Diplura, the Thysanura, and the Ptery- 
gota, but systematists are not agreed as to the relationships of these 
groups. Since the Pterygota are the winged insects, the other forms are 
generally termed collectively the Apterygota. Or, again, the Protura are 
set apart from the others, which are regarded as the true Insecta. The 
Diplura are usually classed with the Thysanura, the two groups then being 
distinguished as Thysanura entotrophica and Thysanura ectotrophica. 

Protura . — ^The proturans are minute creatures resembling insects 
except that they lack antennae. They are more fundamentally distin- 
guished from the other hexapods by the fact that the body does not con- 
tain the definitive number of segments at hatching, there being added 
during growth two segments between the periproct and the preceding 
somite. This postembryonic development of segments is unknown in 
the other hexapods but is usual in the myriapods. The reproductive 
ducts in both sexes of Protura open behind the penultimate segment of 
the adult, as in Chilopoda. Notwithstanding these features, however, 
the Protura appear to be more closely allied to the insects than to the 
myriapods. The body is differentiated into a principal locomotor center, 
the thorax, composed of three segments bearing three pairs of legs, and 
into an abdomen of 12 complete segmental annuli, which has small or 
rudimentary tubular appendages on its first three segments only. The 
thoracic legs end each in a simple clawlike segment (dactylopodite) — ■ 



12 


PRINCIPLES OF INSECT MORPHOLOGY 


another myriapod feature, but one occurring also in some apterygote 
insects and in many pterygote larvae. 

Collembola— The collembolans are small creatures that constitute 
a sharply defined group of hexapods with many distinctive features. 
The abdomen never has more than six segments and is usually provided 
■with three highly specialized appendicular organs. The first of these 
appendages is a large thick tube, the collophore, that projects from the 
ventral side of the first abdominal segment. The third is a leaping organ, 
the furcula, arising from the fifth segment, consisting of a large base and 
two long terminal prongs. When bent forward in repose, the furcula is 
held in place by the small forked second appendage, or tenaculum, arising 
from the third abdominal segment. The genital opening in both sexes 
occurs between the fifth and sixth segments of the abdomen. The repro- 
ductive organs closely resemble those of Protura. 

Diplura {Thysanura entotrophica ). — The members of this group are 
characterized by having the mandibles and maxillae retracted into a 
pouch above the labium. In this respect and in certain other respects 
they are more specialized than the true Thysanura, but the latter are 
more closely related to the Pterygota. The common genera are Cam- 
podea and Japyx. 

Thysanura {Thysanura ectotrophica ) . — The thysanurans have the 
mandibles and maxillae exposed in the usual manner, and they retain 
more complete remnants of abdominal appendages than do the Diplura. 
The coxopodites of the genital appendages in most species are provided 
with gonapophyses that form an ovipositor in the female, which appears 
to be the prototype of the egg-laying organ of pterygote insects. Of the 
two principal families, Machilidae and Lepismatidae, the second shows 
closer aflinities with the Pterygota in the structure and mechanism of the 
mandibles. 

Pterygota . — The Pterygota include all the winged insects and their 
■wingless relatives ; the latter presumably have lost their wings secondarily. 
The wings are lateral extensions of the dorsum of the second and third 
thoracic segments and were fully developed as organs of flight in the oldest 
known fossil insects. During the Carboniferous period, or probably 
earlier, a group of winged insects evolved a mechanism in the wing base 
for flexing the wings horizontally over the back when not in use. The 
descendants of this group (Neopterygota) include the majority of modern 
winged insects, while the more primitive nonwing-flexing insects are 
represented today by only two orders (Odonata and Ephemerida), 
both of which have descended from Carboniferous times but are not 
closely related to each other. 

Ontogenetically the wings are developed during postembryonic 
growth, and a postembryonic metamorphosis, in varying degrees, is 



INTRODUCTION 


13 


common in all the Pterygota. Among the wing-flexing insects, the wings 
in one group are developed during the larval stages within closed pouches 
of the body wall, and the larvae of members of this group have become 
so specialized that their transformation to the adult involves the inter- 
polation of a preimaginal instar, the pupa, between the larva and the 
fully formed adult. Insects of this group are known as the Endoptery- 
gota, because of the internal development of the wings, or as the Holo- 
metabola, in reference to their high degree of metamorphosis. Most of 
the other wing-flexing insects, as well as those that do not flex the wings, 
develop the wings externally in the usual manner of growth of appendic- 
ular organs, and they have in general a simpler metamorphosis that 
allows the nymph or larva to change directly into the form of the adult. 
These insects are distinguished, therefore, as the Exopterygota, or Hemi- 
metabola, but it should be observed that they do not constitute a mono- 
phyletic assemblage of forms, since they include a large group of 
wing-flexing insects and also the nonwing-flexing Odonata and 
Ephemerida. 



CHAPTER II 


GENERAL ORGANIZATION AND DEVELOPMENT 

To understand the structural organization of any animal, it is neces- 
sary to know that animal's history, for no living creature has arrived at 
its present organization by a direct line of development from its beginning. 
Structure generally is an adaptation to function; but many of the organs 
of complex animals have served a series of quite different functions during 
the course of their evolution and, as a consequence, have had their struc- 
ture many times remodeled by way of adaptation to their changing 
function or to new functions. 

The history of an animal cannot be known from the imperfectly 
preserved records of its past. Embryonic development may give us a 
suggestion of phylogenetic evolution, since in a general way the embryo 
repeats the history of its race; but the embryo, for purposes of its own, 
usually digresses much from the ancestral story, and it commonly abbrevi- 
ates its version of the earlier chapters or often omits these parts altogether. 
Fortunately, however, the less highly evolved the final form of an 
organism, the more of its early history is it likely to retain in its ontogeny. 
Hence, we may approximate a reconstruction of the phylogeny of a species 
by filling out the obscure passages in its embryonic story, or by suppl3dng 
those deleted, with material judiciously selected from the facts of 
embryonic development in related forms successively lower in the scale 
of organization. To the evidence derived from embryology, however, 
we must always add that to be deduced from a study of comparative 
anatomy, for in the structure of serially related adult animals we often 
get an insight into the course of evolution more reliable than that to be 
obtained from any other source. 

In the present chapter, therefore, we shall endeavor to build up a 
concept of the fundamental organization of an insect on evidence derived 
from embryology and from a study of the adult structure of other arthro- 
pods, of Peripaius, and of the annelid worms. 

The Germ Cells and the Soma. — The generative cell from which any 
individual of the metazoic animals is produced begins development by 
division. After successive repetitions of division there are soon formed 
two distinct groups of cells in the resulting cell mass ; those of one group 
continue to be germ cells, those of the other group become the somatic 
cells. It is the organization of this second and always larger group of 

14 



GENERAL ORGANIZATION AND DEVELOPMENT 


15 


cells forming the soma, or body of the future animal, that we study in 
ordinary anatomy. 

The purpose of the soma primarily is to give protection to the germ 
cells during the period in which these cells develop to maturity. It 
then becomes obligatory upon the soma, in most forms of animals, to 
bring the germ cells of opposite sexes together in such a manner that a 
sufficient number of them may unite to insure perpetuation of the species. 
In the higher forms of animals, the soma has also taken upon itself the 
responsibility of securing protection to the fertilized eggs and to the early 
developmental stages of the new somata formed by them. Some ani- 
mals acquit themselves of this duty merely by depositing their eggs in 
places where the developing embryos will have a reasonable guaranty 
against destruction; others retain the fertilized eggs and give the embryos 
space for development within their own bodies. Finally, assuming stUl 
greater responsibility, the soma of the more highly endowed types of 
animals charges itself with the duty of protecting and nourishing the 
young during the adolescent period of postembryonic life. In addition 
to fulfilling its many parental functions, and in order to accomplish these 
functions, the soma must also maintain itself as a living unity. 

The methods that the soma has adopted for carrying on its various 
functions are the reasons for its structure; and the continual reorganizing 
of its structure in adaptation to more efficient ways of accomplishing its 
functions has resulted in its evolution from a simple to a complex organ- 
ization. The different methods that the somata of different animals have 
adopted and perfected to meet their obligations are expressed in the many 
forms of life existing today and in those that have existed during the past. 
Structure, vegetative functions, sensitivity to environmental conditions 
and changes, the power of automatic reaction to impinging forces, 
instincts, consciousness, intelligence, and the faculty of making voluntary 
adjustments to external conditions — all these things are properties devel- 
oped and perfected in the soma for the benefit of the germ cells or for the 
somata that are to accompany the succeeding generations of germ cells. 
The soma of the more complex animals cannot reproduce itself ; when its 
purposes are accomplished, its physical elements disintegrate, its vitality 
is reduced, and sooner or later its enemies or adverse circumstances bring 
about its death. 

The germ cells appear to be the reproductive agents of the soma; 
but “reproduction,” so called, is more truly a repeated production of 
somata and germ cells. Each germ cell multiplies by division, and the 
resulting cells undergo a development into spermatozoa or ova, accord- 
ing to their sex, or some of them may become merely nutritive cells 
for the others. The persisting germ cells remain as separate entities, 
except in the union of male and female cells to form a composite cell 



16 


PRINCIPLES OF INSECT MORPHOLOGY 


from whicli the development of both germ cells and soma usually proceeds. 
The ovum alone, however, may produce a complete soma; in insects 
parthenogenesis is of frequent occurrence. The germ cells carry the 
determinants of heredity, called genes, whatever they may be, and 
apparently in most cases receive no direct influence from the accompany- 
ing soma. 

Typical Early Stages of Development, — The egg, being a cell, has 
aU the constituent parts of an ordinary cell; but, since it is destined to a 
much greater activity than any of the body cells, it is provisioned with a 
supply of nutritive material, known as yolk, or deutoplasm, stored in the 
meshes of its cytoplasm. The known facts of comparative embryology 
lead us to believe that the early developmental stages in all animals were 
once essentially the same, though actually they may now be very different 



Fig. 1. — Typical early stages in general embryonic development. A, the blastula 
(diagrammatic). B, C, D, three stages in the differentiation and invagination of the 
endoderm (End), and the formation of the mesoderm (Msd) of a chiton. (From 
Kowalevsky, 1883.) 

in different animals. Some of the differences are clearly correlated with 
the quantity of yolk contained in the egg; others are to be attributed to 
other causes, which may be termed embryonic expediency, for the embryo, 
as well as the postembryonic instars of the animal, departs from the 
ancestral line of development wherever an advantage is offered in so doing. 

The development of the insect presents one of the greatest puzzles 
that the embryologist encounters, and it is certain that the insect embryo 
does not structurally reproduce free-living stages of its ancestry. In 
order to understand insect structure as presented by the embryo, there- 
fore, we must discover the fundamental things in its development that 
underlie those that have been built up to suit the convenience of the 
embryo. 

In the typical, generalized form of animal development, which 
proceeds from eggs containing a minimum quantity of yolk, the first 
division, or cleavage, of the egg cuts the egg into approximately equal 
halves, and the sunilar succeeding divisions soon produce a globular 



GENERAL ORGANIZATION AND DEVELOPMENT 


17 


mass of cells, or blastomeres, called a morula. Then a cavity, the blasto- 
coele, appears in the center of the mass, and the cells become arranged in a 
single layer at the surface. This stage is the blastula (Fig. 1 A); the 
superficial layer of cells is the blastoderm {Bid). The term blast, so fre- 
quently recurring in embryological names, comes from the Greek word 
^'Kaarbs, meaning a bud or sprout. 

The cells on one side of the blastula become distinguished from the 
others by an increase in size (Fig. 1 B). The larger cells then sink into 
the blastocoele cavity, usually as a hollow invagination of the blastula 
wall, and the primitive creature takes on the form of a double-walled sac 
(C). This stage in general embryonic development is the (^asirwZa. The 
blastoderm is now differentiated into an outer layer, or ectoderm {Bed), 
and an inner layer, or endoderm {End), between which is the remnant of 
the blastocoele {Blc). The new cavity is the gastrocoele {Gc), or archen- 
teron, and its external opening is the blastopore {Bpr). 

The endoderm is not necessarily produced in all animals by invagina- 
tion. Gastrulation is often accomplished by an internal proliferation of 
cells from the blastoderm forming an inner cell mass that later becomes 
excavated by a cavity which is the gastrocoele. Invagination and 
proliferation, therefore, are but two variants of the developmental 
process of gastrulation. 

Some of the lowest metazoic animals, such as the Coelenterata, 
never progress beyond the two-layered stage; but in all higher forms the 
two-layered gastrula becomes three layered by the differentiation of a 
middle layer, or mesoblast, from the primary inner layer or from points 
where the outer and inner layers meet. The mesoblast may be produced 
from scattered cells proliferated from the outer layer, and in such cases it is 
called a mesenchyme; but generally it takes the form of a definite cell layer, 
known as the mesoderm. The definitive middle layer may include cells 
produced in both ways. 

In the three-layered stage, the embryo thus has become differentiated 
into an outer ectoderm (Fig. 1 D, Bed), an innermost endoderm {End) 
lining the gastrocoele {Gc), and an intermediate mesoderm {Msd). The 
gastrocoele becomes the stomach of the future animal, the epithelial 
walls of which are formed of the endoderm. The mesoderm, in its 
typical mode of development, is given off laterally in each side of the body 
where the endoderm joins the ectoderm just within the lips of the blasto- 
pore (Figs. 1 D, 3 A, Msd). Animals possessing a mesoderm are typically 
elongate in form and are bipolar, since one end is habitually forward in 
progression; they acquire a dorsoventral differentiation of structure and 
consequently have a bilateral symmetry in their organization. The 
blastopore, originally posterior, as in the free-swimming larvae of certain 
Coelenterata, becomes ventral either by a migration in position or by a 



18 


PRINCIPLES OF INSECT MORPHOLOGY 


forward elongation on the under surface of the embryo. These stages of 
development, which probably took place in the ancestors of the arthro- 
pods, are well illustrated in the larvae of annelid worms and in the 
embryo of Peripatus (Fig. 2). 

Few animals, however, actually follow the simple form of early 
development outlined above; many appear to depart widely from it. 
Yet it is believed that in all cases the actual development is but a modifica- 



Fio. 2. — jEarly stages in the embryonic development of Peripatus capensis. (From Balfour, 

1883.) 

tion of the simple type. The arthropods particularly are aberrant in their 
ontogeny. The student of entomology, therefore, must keep clearly in 
mind the basic course of development in order to understand the depar- 
tures from it that will be encountered in the growth of the insect embryo. 

The Insect Egg. — ^The egg of an insect is usually contained in a shell, 
called the chorion (Fig. 4 B, Cho). The chorion has the appearance and 
texture of the body-wall cuticula, but it is said to be nonchitinous. It is 



Fig. 3. Transverse sections of a young embryo of Peripatus capensis, shoiving the lateral 
mesoderm bands and their coelomic cavities. (From Balfour, 1883.) 

formed in the ovary as a product of the walls of the egg follicle, and it 
bears externally the imprint of the follicular cells. At the anterior 
end of the egg in most cases the chorion is perforated by a minute opening 
or group of pores known as the micropyle {Mi), which permits the entrance 
of the spermatozoa. The delicate cell wall of the ovum becomes the 
vitelline membrane {Vit) of the mature egg. When the egg contains 
much yolk, or deutoplasm (Fig. 4 B), most of its cytoplasmic material 
is distributed in a layer of clear cortical cytoplasm {cpl) about the periphery 



GENERAL ORGANIZATION AND DEVELOPMENT 


19 


of the egg, and in a small mass of nuclear cytoplasm (npl) around the 
nucleus, the remainder being reduced to a network of strands or sheets, in 
the meshes of which is held the yolk material (F). The typical shape of 
an insect egg rich in deutoplasm is elongate oval, which is the usual form 
of the egg within the ovarian tubule (Fig. 287) ; but when extruded the 
egg may take on various shapes, ranging from that of an elliptical globe 
to that of a flattened disc. The chorion may be a smooth shell having 
the form of the egg, or it may assume curious and bizarre shapes. Pro- 
tected eggs are usually of the simple elliptical, oval, or elongate type 
(Fig. 4 B), with one side commonly a little less convex than the other or 
sometimes slightly incurved. A few insects enclose their eggs in cases, or 



Fig. 4. — Two types of insect eggs, sectional views. A, egg of a collembolan. Isotoma 
cinerea, with small quantity of yolk. {From PMliptschenko, 1912.) B, diagram of usual 
type of insect egg containing much yolk. 

oothecae, formed by a secretion discharged from the oviduct or from 
special colleterial glands. 

Cleavage and the Formation of the Blastoderm. — Most insect 
eggs contain such a large quantity of yolk that their size is quite out of 
proportion to the amount of their protoplasmic matter. The ordinary 
insect egg, therefore, to avoid what would be a cumbrous procedure if its 
first cleavage division were to cut through its entire mass, limits its 
activities to the nucleus and the nuclear cytoplasm. This method of 
cleavage in the egg is called meroblastic, in distinction from total cleavage, 
which is holoilastic. 

Not all insect eggs, however, are meroblastic; the eggs of Collembola, 
which have a comparatively small quantity of yolk (Fig. 4 A), divide by 
entire cleavage, and the holoblastic form of division continues until a 
typical morula is formed (Fig. 5). The early development of Anurida 
has been described by Claypole (1898), that of Isotoma by Philiptschenko 
(1912). 

The blastomeres in the morula of Anurida, Claypole says, are prac- 
tically all of equal size (Fig. 5 G, H). In the follovdng stage, however, 
there ensues a period of disintegration in the blastomeres (Fig. 6 A, Blm), 
accompanied by a migration of the nuclei in small masses of cytoplasm 
(a) toward the periphery of the egg. The yolk is thus left behind in the 



20 


PRINCIPLES OF INSECT MORPHOLOGY 


central part of the egg as an inert mass in which the original cell 
boundaries are lost. The migrating nucleated fragments of cytoplasm, 
which are now virtually small cells, divide as they proceed to the 
exterior and arrange themselves in two layers (B) . The outer, continuous 


A 




Fig. 6 . — Example of holoblastic cleavage in an insect egg, Anurida maritima.^ {From 
Claypole, 1898.) A, group of eggs. B-G, successive stages of cleavage resulting in a 
typical morula (H). 


layer is the ectoderm {Ecd), the inner, incomplete layer is the mesoderm 
(Msd), Some of the cells, however, remain behind in the yolk (B, End). 
These cells, according to Claypole, later form the stomach epithelium in 
Anurida and are therefore endodermal. 



Fig. 6. — Developmental stages of Anurida maritima following holoblastic cleavage of 
the egg. (From Claypole, 1898.) A, disintegration of the blastomeres. B, differentia- 
tion of the germ layers. 


In Isofoma, as described by Philiptschenko, the holoblastic cleavage of 
the egg produces a compact morula in which the cleavage cavity disap- 
pears. Then each of the blastomeres divides into a smaller outer cell 
containing the nucleus, and into a large, nonnucleate inner sphere which 
retains the yolk. The small nucleated cells migrate outward between 
the yolk spheres and come to rest at the surface of the egg, where they 



GENERAL ORGANIZATION ANB DEVELOPMENT 


21 


form at first a single continuous layer, which is the blastoderm. The 
blastoderm cells later differentiate into an outer layer of ectoderm and an 
inner layer, which, in Isotoma, Philiptschenko claims is the common 
rudiment of the future mesoderm and endoderm. 

In meroblastic eggs the nucleus and its containing mass of cytoplasm, 
situated in the interior of the egg, behave in the manner of an ordinary 
cell. By repeated division, together with division of the daughter cells 
(Fig. 7 A), they produce an increasing number of cleavage cells (CCls), 
which migrate to the periphery of the egg (B) and there become disposed 
in a single layer just beneath the vitelline membrane (B, C). The 
cortical cytoplasm then unites with the cleavage bodies to form definitive 
cells, which constitute the blastoderm (C, D, Bid). In this stage, the 
insect embryo derived from a meroblastic egg is equivalent to the blastula 



Fig. 7. — Diagrams of meroblastic cleavage and the process of blastoderm formation typical 
of insect eggs having a large quantity of yolk. 

in the generalized form of development (Fig. 1 A), but the cells of the 
blastoderm are all contained within the original egg wall, or vitelline 
membrane. 

Within the blastoderm is the mass of yolk material held in the meshes 
of the inner cytoplasm of the egg, and usually a few yolk cells (Fig. 7 C, 
YCls). The latter are nucleated cytoplasmic masses derived from the 
egg nucleus that did not take part in the formation of the blastoderm. 
In addition to these primary yolk cells, other cells are often found in the 
yolk after the completion of the blastula, which are said to be proliferated 
from the blastoderm. These cells, some investigators claim, are endo- 
derm cells that accomplish a partial digestion of the yolk before the 
embryonic stomach is formed, and for this reason they have been named 
vitellophags. It appears, as we shall presently see, that the walls of the 
stomach in some insects are formed in part or entirely from migratory 
endoderm cells proliferated into the yolk at an early stage of development. 

The Germ Cells. — The germ cells of many insects become recognizable 
at the time the blastoderm is being formed. In the collembolan Isotoma, 



22 


PRINCIPLES OF INSECT MORPHOLOGY 


Philiptschenko (1912) says, the germ cells first appear as a small, compact 
group of cells in the yolk near the posterior end of the blastula (Fig. 8 A, 
GCls). In insects with meroblastic cleavage the germ cells are usually 
observed first at the posterior end of the blastoderm (Fig. 7 C, GCls), 
where they lie in a differentiated protoplasmic area called the germ tract 
(Keimbahn) and in some cases protrude somewhat from the surface of the 



Fig. 8. — Early differentiation of the germ cells (OCls) from the somatic cells. A, 
blastula of Isotoma cinerea. (From Philiptschenko, 1912.) B, blastula of Drosophila 
melanogaster with germ cells segregated at posterior pole. (From Huettner, 1923.) 

blastoderm (Fig. 8 B, GCls). The protoplasm of the germ tract is often 
marked by the presence of dark-staining granules, and certain writers 
have suggested that it is some peculiar quality of the germ-band proto- 
plasm (Huettner, 1923), or even of the granules themselves (Hegner, 
1914), that differentiates as germ cells whatever cleavage cells happen to 
wander into the germ tract. Huettner shows that even at this early stage 



Fig. 9. — Diagrams of the formation of the germ band (OB) on the ventral side of the blasto- 
derm. A, cross section. B, longitudinal section. 

there is a difference in the nuclei between the germ cells and the blastoderm 
cells. 

The germ cells undergo a short period of activity during which they 
increase slightly in numbers, and then they enter a long period of rest 
while the somatic cells are developing. The germ cells move into the 
interior of the body and are finally lodged in the dorsal part of the meso- 
derm that forms the ovaries or testes (Fig. 22, GCls). 

The Germ Band. When the insect blastoderm has completely sur- 
rounded the egg, the creature is in the blastula stage (Figs. 1 A, 7 C). 
Its cleavage cavity, or blastocoele, is filled with the yolk. The first 


GENERAL ORGANIZATION AND DEVELOPMENT 


23 


differentiation that leads to the formation of the specific embryo consists 
of a thickening of the cells in that wall of the blastula which is to be the 
ventral side of the future insect (Fig. 9). This region of enlarged cells 
(GB) appears on the surface as an opaque oval or elongate area, which 
is known as the germ band, germinal disc, or primitive streak (Keimscheibe, 
Keimstreif). The blastoderm is now differentiated into an embryonic 
area {GB) and an extraembryonic field (DBl), the latter being the dorsal 
part of the blastoderm, composed of small, flat, often attenuate cells, 
termed the dorsal blastoderm, or serosa. 

Formation of the Inner Germ Layers. — This subject is a most difficult 
one to understand in insect embryology, if by “understanding” we mean 
an interpretation of the Imown facts of the formation of the germ layers 
in insects according to the terms of the general gastrulation theory. 
The crux of the difficulty lies in the fact that the apparent gastrulation 
of the insect embryo gives rise directly to a cell layer that forms the tissues 
which are of mesoblastic origin in other animals, while the epithelium 
of the mesenteron, which should be the endodermal wall of the gastrula- 
tion cavity, appears in most cases to be derived independently from the 
blastoderm. 

In the typical and general mode of development, as we have seen, the 
embryo becomes two layered by the invagination of one side of the 
blastoderm (Fig. 1 C) ; but it was noted also that the invagination method 
of gastrulation may be modified into a process of internal proliferation 
of cells from a closed blastopore area of the blastoderm. The typical 
embryo next becomes three layered by the generation of a mesoderm 
from invaginations or proliferations of cells from the blastoderm just 
within the lips of the blastopore where the ectodermal and endodermal 
layers meet (Figs. 1 D, 3 A, Msd). It is evident, therefore, that the 
normal relations of the endoderm and the mesoderm might be obscured 
if the endoderm were formed in part or entirely by the proliferation of 
scattered cells, for it would then appear that gastrulation produces only 
the mesoderm. A discontinuous formation of the endoderm commonly 
takes place in the arthropods, and this condition has led to much mis- 
understanding of the true relations of the germ layers; but if we keep in 
mind the important fact that it is the function of the endoderm to absorb 
and digest the yolk, we need not be disconcerted by the various ways it 
adopts for fulfilling its destiny. 

In some of the Crustacea an archenteron is formed by typical gas- 
trulation in which an invaginated endoderm becomes directly the stomach 
of the mature animal. In such cases the endoderm absorbs the yolk mass 
and passes it into the archenteric cavity, from which it is drawn for use 
by the growing tissues. In certain other crustaceans the walls of the 
archenteron disintegrate, and the liberated endoderm cells migrate into 



24 


PRINCIPLES OF INSECT MORPHOLOGY 


the yolk, whence they later emerge and arrange themselves again in an 
epithelial layer, but this time surrounding the yolk to form the walls of 
the definitive stomach. In other cases, again, though the major part of 
the endoderm is fragmented, two flat masses of its cells remain intact 
beneath the yolk, and from these rudiments the mesenteron epithelium 
is later regenerated. In this case the endoderm cells lost to the yolk 
remain there as vitellophags. These various modifications of mesenteron 
formation by the endoderm are generally correlated with the amount 
of yolk in the egg. 

When we turn now to the insects, we find, so far as known, that the 
primitive method of forming the mesenteron by the typical invagination 



Fig. 10. — Diagrams suggesting the evolution of germ-layer formation in pterygote 
insects. A, primitive gastrula with open blastopore and lateral bands of mesoderm. 
B, blastopore closing. C, blastopore closed, the archenteron in state of disintegration. 
D, most of endoderm cells dispersed in yolk as vitellophags {Vph), leaving only a ventral 
remnant {End) intact. E, differentiation of the ventral endoderm remnant into anterior, 
intermediate, and posterior mesenteron rudiments {AMR, IMR, PMR). 


process of gastrulation does not exist. We must presume, then, that, 
having become impracticable owing to the amount of yolk in the insect 
egg, it has been completely eliminated from insect ontogeny. Various 
stages may be traced, however, in the fragmentation of the major part 
of the endoderm, and the differentiation of its ventral remnant into 
several groups of cells that regenerate the embryonic stomach, or mesen- 
teron. These groups of cells are the so-called mesenteron rudiments. 

In the insect embryo the blastopore is never an open aperture, but the 
blastopore area, as indicated by the region from which the endoderm and 
mesoderm are formed, becomes elongated forward on the ventral side of 
the embryo, appearing sometimes as a groove (Fig. 23 A, Bpr) comparable 
with the long, open, slitlike blastopore of Peripatus (Fig. 2 B). We may 



GENERAL ORGANIZATION AND DEVELOPMENT 


25 


assume, therefore, that gastrulation and the formation of the mesoderm 
originally took place along this line (Fig. 10 A). With the closure of the 
blastopore, then, the mesoderm comes to be the first layer immediately 
within the blastopore area (B, C, Msd). In this case it is clear that the 
endoderm {End) would appear to be generated from the mesoderm, and 
that by complete disintegration it might take the form of cells scattered 
through the yolk (C) or, on the other hand, a ventral remnant of it might 
be preserved beneath the yolk (D). In the light of this concept we can 
understand many of the processes of mesoderm and endoderm formation 
described by different VTiters on insect embryology, the discrepancies 
between which have seemed irreconcilable. 

The Mesoderm. — In the Collembola, according to Claypole (1898) 
and Philiptschenko (1912), the first-formed inner layer is the mesoderm. 
It is produced by a general tangential division of the blastoderm cells, 
the division taking place either at the time the cleavage cells reach the 
surface of the egg (Anurida, Claypole) or afterwards {Isotoma, Philip- 
tschenko). This method of mesoderm formation may be conceived as an 
unlocalized form of proliferation from the blastoderm. 

In the Thysanura the mesoderm is formed by a localized proliferation 
of cells from the midventral line of the blastoderm, the area of prolifera- 
tion being sometimes marked externally by a pit or a groove. In Cam- 
podea, according to Uzel (1897), and in Leptsma, as described by Heymons 
(1897), the mesoderm proliferation arises at a central point of the blasto- 
derm, the cells spreading out into a disc as they are given off. Heymons 
does not believe that the limited extent of the proliferation area here 
represents a polar gastrulation; he attributes it rather to the small size 
of the egg and to the oval shape of the germ band. 

In the Pterygota the surface of the usually elongate germ band 
becomes differentiated into a median area, or middle plate (Fig. 11 A, MP), 
and into two lateral areas known as the lateral plates {LP). From the 
middle plate is formed the mesoderm, usually in one of three ways; in 
some cases by an infolding or invagination of the middle plate (B, C), in 
others by an overgrovdh of the median plate by the lateral plates (D, E), 
and in stUl others by a proliferation of cells from the inner surface of the 
middle plate (F). In any case the formation of the mesoderm evidently 
represents a modified gastrulation, from which the endoderm appears to 
have been eliminated; but, as we shall presently see, the middle plate 
more probably represents a common mesoderm-endoderm rudiment, 
since a part of the endoderm is usually generated from the mesoderm or in 
close association with it. 

The Endoderm. — 'V\Tiere the processes of gastrulation take place by 
proliferation from the area of a closed blastopore, it is clear, as we have 
already noted (Fig. 10 C), that the endoderm, if produced in connection 



26 


PRINCIPLES OF INSECT MORPHOLOGY 


with the mesoderm, will appear to arise from the latter. In the collem- 
bolan Isotoma, Philiptschenko (1912) says, the endoderm is generated 
from that part of the inner germ layer along the midventral line of the 
embryo that does not take part in the formation of the mesoderm somites 
(see Fig. 19 A). The endoderm tissue consists at first of three groups of 
cells — an anterior group, which is the anterior mesenteron rudiment, a 
posterior group, which is the posterior mesenteron rudiment, and a median 
strand of loosely associated cells forming an intermediate mesenteron 
rudiment lying beneath the yolk and connecting the two terminal rudi- 
ments. The definitive embryonic stomach, or mesenteron, is produced 
by the multiplication of cells in each of the rudiments and the growth of 



Fig. 11. — Diagrams showing three methods of mesoderm formation in pterygote 
insects. A, cross section of egg with germ band differentiated into middle plate {MP) 
and lateral plates (iP). B, C, the middle plate invaginated to form the mesoderm. D, 
E, the middle plate overgrown by the lateral plates. F, mesoderm proliferated internally 
from the middle plate. 


the cells around the yolk until they form an epithelial sac enclosing the 
latter. Here, therefore, it is clear that the primitive inner layer is a 
mesoderm-endoderm layer, since the endoderm is differentiated from its 
entire length. A similar condition is said by Strindberg (1913) to exist 
in termites {Eutermes) and in ants, where the endoderm arises by a 
proliferation of cells from the entire length of the mesoderm (or meso- 
derm-endoderm rudiment) ; these cells lie beneath the yolk and eventually 
surround it to form the walls of the mesenteron. 

There is reason to believe that the enclosure of the yolk by endoderm 
cells derived from the mesoderm-endoderm rudiment does not strictly 
represent the process of gastrulation in insects, but that it is a secondary 
regeneration of the mesenteron taking place subsequent to an earlier 
fragmentation of the walls of the true gastrulation cavity. This con- 
clusion is to be deduced from the observation of several investigators. 



GENERAL ORGANIZATION AND DEVELOPMENT 


27 


According to Claypole (1898), the endoderm cells of the collembolan 
Anurida originate during cleavage of the egg, but they remain in the yolk, 
while the blastoderm cells that give rise to the ectoderm and mesoderm 
are migrating to the surface. Finally, however, the endoderm cells 
engulf the yolk and then arrange themselves to form the definitiA^e 
mesenteron, which contains practically no yolk except that in the bodies 
of its cells. 

A similar method of mesenteron formation is described by Heymons 
(1897) for Lepisma. In this insect, Heymons says, the mesenteron 
epithelium is generated from cells found at an early stage of develop- 
ment within the yolk. These cells later migrate to the periphery of the 
yolk mass where they become aggregated into scattered islands that 
grow in extent by multiplication of their cells until eventually they sur- 
round the yolk and constitute the wall of the definitive mesenteron. 

In the Odonata the mesenteron is said by Tschuproff (1904) to be a 
composite organ made up of cells derived from three distinct sources, its 
anterior and posterior parts being formed from cells proliferated from 
the inner ends of the stomodaeum and proctodaeum (Fig. 13, Siam, Proc), 
while its middle part is formed of cells that migrate outward from the 
yolk. The cells derived from the yolk Tschuproff regards as the true 
endoderm; the others she claims are ectoderm cells. This conclusion 
concerning the nature of the anterior and posterior parts of the mesen- 
teron is not necessary, for it is well known that the anterior and posterior 
mesenteron rudiments derived from the middle plate of the embryo may 
be carried inward by the stomodaeal and proctodaeal invaginations; 
but the observations of Tschuproff, if true, do show that the Odonata 
present a gradient condition in the endodermal activities between one in 
which the mesenteron is re-formed from a disintegrated archenteron and 
one in which it is regenerated from anterior and posterior groups of endo- 
dermal cells. 

It is interesting to note that Eastham (1927) finds in Pieris rapae a 
median proliferation of cells from the middle plate of the embryo (Fig. 12, 
MPCls); these cells take no part in the mesoderm formation but pass 
into the yolk and there disintegrate. The proliferation of these cells, 
Eastham says, begins at the anterior end of the middle-plate region and 
“continues along the middle line of the embryo from before backward,” 
until there is formed “a median track of proliferation passing almost from 
end to end of the embryo.” T^Tiile Eastham apparently does not 
definitely commit himself to the view that these ceUs are endodermal, 
he mentions ha^dng observed in one series of sections a continuity at an 
early stage of development between them and the posterior mesenteron 
rudiment. It seems clear, however, that the line of proliferating cells 
must represent the true gastrulation of the insect in a rudimentary and 



28 


PRINCIPLES OF INSECT MORPHOLOGY 


disintegrated state. The cells, Eastham says, “in all probability help 
in the liquefaction of the yolk, rendering the latter capable of being 
assimilated by the germ band.” These " middle-plate cells ” that wander 
into the yolk are evidently endodermal vitellophags. 

In the majority of pterygote insects, mesenteron rudiments have 
been found only at the two ends of the mesoderm (Fig. 13 B, AMR, 
PMR ) ; but, as we have observed, it is claimed by Strindberg that in the 
termites and in ants the anterior and posterior rudiments are connected 
by an intermediate rudiment (A, IMR), thus duplicating the condition 
described by Philiptschenko in Isotoma. It is evident, therefore, that the 
usual condition, in which the mesenteron is formed from anterior and 
posterior rudiments alone, has resulted from a suppression of the inter- 



Fig. 12. — Proliferation of cells (MPCls) from tlie middle plate of the germ band of Pieria 

rapae. (From Eastham, 1927.) 

mediate rudiment, or from the migration of its cells into the yolk, as in 
Pieris, and that the two persisting end rudiments represent the final 
remnants of a disintegrated archenteron (Fig. 10 C). In the Lepidoptera, 
according to Schwangart (1904) and Eastham (1927), the anterior and 
posterior mesenteron rudiments are formed in continuity with the meso- 
derm ; but in other insects the two rudiments often appear to arise either 
directly from the blastoderm at the two ends of the mesoderm (Fig. 
10 E) or from the walls of the ectodermal stomodaeal and proctodaeal 
invaginations (Fig. 13). 

Resume of Gastrulation . — ^The formation of a stomach by the ancestral 
process of open gastrulation (Fig. 10 A) becomes impracticable in embryos 
containing a large quantity of yolk. In insect phytogeny the blastopore, 
after elongating forward on the ventral side of the embryo, has become 
permanently closed (B, C) and gastrulation takes place by proliferation 
instead of by open invagination, the cells thus formed migrating sepa- 
rately into the yolk. This process represents a fragmentation of most of 
the wails of the archenteron (C), an embryonic device by which the 



GENERAL ORGANIZATION AND DEVELOPMENT 


29 


endoderm cells may surround the yolk. In some of the Apterygota the 
definitive stomach (mesenteron) is said to be regenerated by the scattered 
endoderm cells; in Odonata it is perhaps regenerated in part by these 
cells. In some of the Apterygota and in most of the Pterygota, however, 
the proliferated endoderm cells remain as vitellophags in the jmlk (D, 
Vpli), and the definitive stomach is regenerated from a ventral remnant of 
the endoderm (C, D, End) that 
remains in contact with the meso- 
derm below the yolk. This 
ventral endoderm remnant may 
consist of a continuous band of 
cells coextensive with the meso- 
derm (E, AMR, IMR, PMR), or 
it may be reduced to anterior 
and posterior groups of cells, 
which are the usual anterior and 
posterior mesenteron rudiments 
(AMR, PMR). Throughout the 
entire course of the life cycle 
of most insects the endodermal 
epithelium of the mesenteron is 
subject to disintegration and 
regeneration. 

The Alimentary Canal. — The 
stomach, or mesenteron, of the 
insect embryo is clearly not the 
primitive archenteron, though 
when completed it is a sac com- 
posed of endoderm cells. As we 
have just seen, it may be formed 
by the reassembling of the primi- 
tive endoderm cells scattered in 
the yolk, or in part from such cells 
and in part from ventral endoderm 
remnants. In most insects, however, it is regenerated entirely from a 
median ventral remnant of the archenteron, including anterior, inter- 
mediate, and posterior rudiments (Fig. 13 A, AMR, IMR, PMR) or more 
commonly from anterior and posterior rudiments only (B, C, D, AMR, 
PMR). The cells of these mesenteron rudiments multipl}’- and extend 
over the surface of the yolk (C) until those proliferated from opposite 
ends meet and finally constitute a complete epithelial sac containing the 
^mlk (D) . The sac is later enswathed by sheets of muscle tissue and even- 
tually becomes the definitive stomach, or vcniricidus, of the adult insect. 









Fig. 13. — Diagrams shon'ing the formation 
of the definitive alimentary canal in pterygote 
insects. A, the germ layers in longitudinal 
section, including the three regenerative 
endodermal rudiments of the mesenteron 
(AMR, IMR, PMR). B, C, D, envelopment 
of the yolk by the growth of the mesenteron 
rudiments to form the stomach (Menl), and the 
ingrowth of the ectodermal stomodaeum 
(Slom) and proctodaeum (Proc). 



30 


PRINCIPLES OF INSECT MORPHOLOGY 


The complete alimentary canal (Figs. 13 D, 14) comprises, in addition 
to the stomach, anterior and posterior sections derived as secondary 
invaginations from the ectoderm. These parts, known respectively as 
the stomodaeum (Stom) and proctodaeum (Proc), later open into the 
mesenteron and thus give the alimentary canal its final form of a complete 
tube extending through the body (Fig. 20) . The embryonic development 



Fig. 14. — Median longitudinal vertical section of a mature male embryo of the honey bee 
surrounded by the serosa and chorion. {From Pclrunkevilch, 1901.) 

of the alimentary canal, however, may entirely ignore the phylogenetic 
order of events, for it is often found that the stomodaeal and proctodaeal 
invaginations are formed prior to the generation of the mesenteron. 
The rudiments of the latter may then be carried inward by the ectodermal 
ingrowths, in which case the mesenteron appears to be generated from 
the opposing ends of the ectodermal parts of the alimentary canal. 



Fig. 15. — Diagrams suggesting the evolution of the synaptic sensory-motor nervous 
system. A, the primary sensory nerve cells (SCI') located in the ectoderm and connected 
directly -with muscles (Md). B, the primary nerve cells internal (now motor cells) and 
stimulated indirectly by nerves from a second set of sensory cells (SCI") in the ectoderm. 

Origin of the Nervous System. — All nerve tissue originates in the 
ectoderm. Nerve cells are ectodermal cells in which the properties of 
irritability and conductivity are highly developed. If their inner ends are 
connected with the motor tissues, the primary nerve cells (Fig. 15 A, SCI') 
become the agents for transmitting external stimuli to the motor elements 
(Mel), and thus they may directly incite the latter to action. In most 
animals, however, the nerve cells connected with the effector mechanisms 
sink beneath the surface of the ectoderm where they become shut off 


GENERAL ORGANIZATION AND DEVELOPMENT 


31 


from direct contact with the outer world (B, SCls'). The sunken nerve 
cells must then be themselves stimulated through the agency of other 
nerve cells {Scl”) that retain surface relations. In this way the general 
nervous system of complex animals comes to consist functionally of a 
motor system lying entirely within the body and of a sensory system main- 
taining, on the one hand, a connection with the exterior and, on the other, 
establishing connections with the motor system (B). 

In the annelids and arthropods the motor nerve cells are contained 
mostly in two more or less closely associated tracts of nerve tissue forming 
the ventral nerve cord (Fig. 16 C, VNC) Ijdng along the midventral line of 
the body. It is probable that this double median nerve cord has been 



Fig. 16. — Diagrams showing the probable evolution of the annelid-arthropod type of 
central nervous system from a preoral nerve center (Arc) and two latoroventral nerve 
cords {NC). 

evolved from two lateroventral bands of ectodermal nerve cells forming 
two primitive lateral cords (A, B) which have approached each other 
medially. In the annelids the two cords are united anteriorly with a 
small nerve mass, or primitive brain, the so-called archicerebrum (Arc), 
lying in the anterior end of the body primarily before the mouth. 

In the ontogeny of insects most of the nerve tissues originate at an 
early stage of development from two longitudinal thickenings of the 
ectoderm near the midline of the germ band, known as the neural ridges 
(Fig. 17 A, NIR), between which is a median neural groove (NIG). From 
the inner surfaces of the ridges are proliferated longitudinal rows of large 
cells, the neuroblasts (Nbl), which are the primarj’- nerve cells of the future 
nerve cords. A median row of neuroblasts also is formed above the neural 
groove. The neuroblasts multiply by division and produce three strands 



32 


PRINCIPLES OF INSECT MORPHOLOGY 


of ganglionic cells (B, GngCls). Thus the definitive ventral nerve cord 
of insects appears to take its origin from two primitive lateral cords of 
nerve cells (LC) and a median cord {MC). The nerve cells formed from 
the neuroblasts send out processes which become the nerve fibers. Some 
of the fibers remain in the nerve cords where they establish communica- 
tion between the nerve cells; others extend outward to the other body 
organs and constitute the motor nerves. 

The sensory nerves of insects proceed inward from sensory cells of 
the ectoderm to the ventral nerve cord and here form intimate associa- 
tions with the motor cells. Little 
is definitely known, however, con- 
cerning the actual development of 
the sensory nerves in arthropods. 

When segmentation takes place 
in the body, the nerve cords are 
differentiated into segmental 
regions, or ganglia (Fig. 16 B, Gng), 
containing the nerve cells and into 
intersegmental connectives {Con) 
composed of nerve fibers. The two 
ganglia of each segment become 
connected crosswise by fibrous 
commissures {Com) and finally are 
so closely approximated and bound 
to each other that they constitute 
a single segmental ganglion (C), 

though the intersegmental con- 
Fig. 17. — Embryonic origin of the ventral ,. • j 1. 1 rnt. 

nervous system from the ectoderm. {Dia- nectives remain double. Ine 

grams based on figures from Wheeler, 1891.) successive paired ganglia in the 
Ecd, ectoderm; GngCls, ganglion cells; LC, „ , i i • j. 

lateral nerve cords; MC, median nerve cord; lUture nead region 01 the inseCt 
Nbl, neuroblasts; NIG, neural groove; NIR, always COaleSCe Ul tWO grOUpS .' those 
neural ridge. anterior part of the head 

unite with the primitive archicerebrum to form the brain (Fig. 14, Br), 
which lies above the stomodaeum {Stom)] those of the posterior part 
compose the suboesophageal ganglion {SoeGng), which lies below the 
stomodaeum. 

The Embryonic Coverings. — ^The embryos of most insects become 
separated in one way or another from contact with the egg shell during a 
part or all of the period of their development. We may distinguish four 
methods of separation, namely, (1) by invagination of the embryo, (2) 
by involution of the embryo, (3) by the formation of cellular protective 
membranes, (4) by the production of cuticular coverings. The last 
constitutes a distinct process, probably allied to moulting; the others 



MC 



GngCls g Nbl 



GENERAL ORGANIZATION AND DEVELOPMENT 


33 


intergrade, and the second and third are possibly derivations from the 
first. 

Invagination of the Embryo. — In the Apterygota the germ band 
doubles upon itself ventrally at an early stage of development and sinks 
into the egg or the yolk in the form of an inverted U, in which position it 
may remain until the embryo is 
almost fully formed (Fig. 18 A, 

B). This method of development 
is characteristic of many other 
arthropods, and its retention in 
the apterygote insects apparently 
has a phylogenetic significance. 

In the Collembola (Fig. 18 A) 
the embryo fills the entire egg 
after the ventral flexure takes 
place. In Lepisma, as described 
by Heymons (1897), the embryo 
begins its development on the 
surface of the egg in the usual 
manner, but soon it curves ven- 
trally and sinks into the yolk (B). 

As the submergence increases, a 
part of the surrounding extraem- 
bryonic blastoderm is turned in 
also, and the mouth of the cavity 
contracts to a small pore, but 
it does not entirely close. Later, 
the Lepisma embryo partially 
emerges and completes its devel- 
opment with its head and thoracic 
parts again on the surface (C). 

Involution of the Embryo. — In several other orders of insects, including 
the Odonata, some of the Orthoptera, and the Homoptera, the embryo, 
which begins its development on the surface of the egg (Fig. 18 D), turns 
into the yolk tail end first, by a process of revolving on its transverse axis, 
and stretches out within the egg in a reversed and inverted position (E). 
The revolution of the embryo carries a part of the extraembryonic blasto- 
derm into the yolk, and when the reversal of the embryo is complete the 
opening into the embryonic cavity is closed. There is thus differentiated 
from the extraembryonic blastoderm an inner part lining the embrjmnic 
cavity, which is the amnion {Am), and an outer part, or serosa (Ser), 
which surrounds the egg. Before the odonate or hemipteran embrjm 



Ser- 

F ' I 

Fig. is. — T hree methods by which the 
insect embryo may become separated from the 
egg shell. A-C, invaoination of the embryo 
(A, Isoloma cincrca, from Philiptschcnko 1912; 
B, C, Lepisma, from Heymons, 1897). D-F, 
involution of the embrj’O and its return to the sur- 
face of the egg, diagrammatic. G-I, formation 
of cellular covering membranes, diagrammatic. 



34 


PRINCIPLES OF INSECT MORPHOLOGY 


completes its development, it reverts again to the surface by a counter- 
revolution that restores it to its original position (F). 

Cellular Embryonic Membranes.— The more usual type of embryonic 
covering is that produced by outgrowths of the blastoderm forming a 
cellular sheath over the embryo. In typical cases of this kind folds 
from the extraembryonic part of the blastoderm grow out around the 
ends or along the sides of the germ band (Fig. 18 G, b), the edges of which 
come together (H) and unite beneath the embryo (I) . The corresponding 
layers of the opposite folds then become continuous, and the embryo is 
shut in beneath two cellular membranes, the outer of which is the serosa 
{Ser ) , the inner the amnion {A m ) . Usually the amnion and serosa remain 
in contact over the ventral surface of the embryo; but in some cases, 
particularly in the Lepidoptera, the embryo and the amnion sink into the 
yolk, an,d a part of the yolk then fills the space betAveen the amnion 
and the serosa, producing a submerged condition of the embryo. In the 
honey bee, according to Nelson (1915), the embryonic covering consists of 
a single layer of cells, formed by two outgrowths from the serosa along 
the edges of the germ band, which eventually meet and unite beneath the 
embryo. 

The cellular embryonic membranes usually disappear before the 
embryo reaches maturity. In most cases they separate in a longitudinal 
cleft beneath the embryo, and the resulting folds are carried upward as 
the dorsal blastoderm contracts above the expanding lateral walls of the 
growing embryo. The cells of the dorsal blastoderm and the amniotic 
folds eventually sink into the yolk on the dorsal side of the egg, where they 
are finally absorbed. In the honey bee, the single embryonic membrane is 
said to persist until the time of hatching, when it is broken up by the 
movements of the young larva in the egg. 

Embryonic coverings formed of blastodermic folds are characteristic 
of insects; a similar structure is said to be found in other arthropods only 
in the scorpions. 

Cuticular Embryonic Membranes . — Embryonic coverings of a non- 
cellular structure are of common occurrence in all groups of the Arthro- 
poda. These membranes, apparently, are of the nature of cuticular 
exuviae, for it is stated by Campbell (1929) that the embryonic invest- 
ment of the cockroach is a chitinous tissue. Their separation from the 
embryo, therefore, may be regarded as an embryonic moult. The 
embryonic membranes are shed at the time the young arthropod leaves 
the egg or shortly thereafter. 

Cuticular embryonic coverings occur in all insects with incomplete 
metamorphosis and have been observed in some holometabolous forms. 
In Anurida, as recorded by Claypole (1898), three distinct membranes 
are given off from the entire surface of the embryo before the appendages 



GENERAL ORGANIZATION AND DEVELOPMENT 


35 


are formed and before the embryo curves into the egg. In most insects, 
however, only one cuticular embiyonic membrane is known to exist. 
In some cases, as in the aphids, the membrane has the form of a simple 
sac enveloping the embryo, but in other insects the sac may be provided 
with pouches that individually ensheath the appendages. Evidentl}’-, 
then, the period of embryonic growth at which the moult is given off 
varies in different insects. A similar embrimnic covering occurs in 
Arachnida, Crustacea, ChUopoda, and Diplopoda, in some cases envelop- 
ing the embryo as a cuticular sac, in others having close-fitting extensions 
over the appendages. 

Segmentation of the Body. — The first important step in the evolution 
of the annelids and arthropods was that by which the bod}’’ lost its unity 
of structure and became broken up into a series of parts, or somites. 
The somites are in general called body segments, but in the embryo 
they are more specifically termed metameres. 

In ontogenetic development, metamerism is usually described as 
originating in the mesoderm, the segmental regions bemg first marked by 
a closer massing of the mesodermal cells, which thus appear as a series of 
opaque areas in the germ band alternating with more transparent lines. 
It is pointed out by Eastham (1927, 1930), however, that in the embrj’^o 
of Pieris external segmentation, shown by indentations of the ectoderm, 
precedes the formation of the mesodermal somites. In the larvae of the 
annelid worms, which develop two lateral bands of mesoderm tissue, 
the segmental masses of mesodermal cells soon become hollowed by the 
formation of clefts between their dorsal and ventral cells, there being thus 
produced a series of paired mesodermal pouches that eventually e.xtends 
through the entire length of the body. These pouches are the coelomic 
sacs. As development proceeds, the coelomic sacs enlarge upward in 
the sides of the body and either remain thus as definitive segmental body 
cavities or unite in a common body cavity coextensive with the length of 
the animal. 

In the Arthropoda the mesoderm becomes distinctly segmented, but 
coelomic cavities are not so definitely formed as in the Annelida, and in 
many cases they are entirely absent. Where they appear they generally 
take the form of small clefts in the lateral parts of the mesoderm (Fig. 
19 A, Coel, Cocl), but the series on each side soon becomes converted into 
a coelomic tube by the disappearance of the partitions, and the irmer 
walls of the tubes break down (B), so that the segmental cavities are at 
best but evanescent. 

Segmentation in the arthropods, as in the annelids, is not a process of 
“budding.” The somites are formed alwa 5 "s anterior to a small terminal 
piece, the telson, or pcriproct (Fig. 20, Prpt), which contains the anus and 
gives rise to the proctodaeal invagination. Even in arthropods in which 



36 


PRINCIPLES OF INSECT MORPHOLOGY 


segmentation is not complete at hatching, the segments added during 
postembryonic development are formed in the region immediately anterior 
to the periproct. Embryonic segmentation usually appears first in the 
gnathal region and proceeds posteriorly, though there is often much 
irregularity in the sequence of the newly forming somites. The seg- 



Fiq. 19. — Formation of the coelomia sacs and the ventral nerve cord in the embryo of 
Forficula. (From Heymons, 1895.) A, cross-section of young embryo showing ooelomic 
clefts (Coel) in the mesoderm. B, later stage in which the ventral nerve cord is differenti- 
ated from the ectoderm, and the ooelomic sacs open into the epineural sinus (EpnS) of the 
haemocoele. 


mented area of the trunk, however, finally extends from the mouth to the 
periproct, but, since the mouth lies a short distance behind the anterior 
pole of the embryo, there is always an unsegmented preoral region of the 
animal, which is the prostomium (Fig 20, Prst). The prostomium is 



Fig. 20. — Diagram of the fundamental structure of a segmented animal of the annelid- 
artoopod type. An, anus; Arc, archicerebrum; Ment, mesenteron; Mth, mouth; Prpt, 
periproct; Proc, proctodaeum; Prst, prostomium; Stom, stomodaeum; VNC, ventral nerve 
cord. 


represented in the arthropod embryo probably by the cephalic lobes 
(Fig. 23, Prc ) ; in the annelid trochophore larvae it is the large preoral 
swelling of the trunk. The segments between the prostomium and the 
periproct are the true somites, of which in the insects there are at least 
18 when segmentation is complete. The insect embryo attains its defini- 



GENERAL ORGANIZATION AND DEVELOPMENT 


37 


tive segmentation before leaving the egg, but in the Protura the last two 
somites are formed just anterior to the periproct during postembryonic 
life. 

Since segmentation affects primarily the mesoderm and the ectoderm, 
but not the endoderm, the alimentary canal is never segmented ; but the 
metamerism of the body influences most of the organs subsequently 
formed, such as the nervous system, the circulatorj’^ organs, the body 
appendages, the tracheal invaginations, and the reproductive organs. 

The Segmental Appendages. — ^The acquisition of segmentation was 
undoubtedly in itself an important event in the evolution of the seg- 
mented animals toward a higher development because of the increased 
facilities of movement that came with it ; but the advantages of metamer- 
ism were not fully available until the segments became equipped vith 
movable appendages. 

The segmental appendages of arthropods are developed in the embrim 
as hollow, paired outgrowths of the body wall, appearing soon after 
segmentation, on the lateroventral parts of the germ band (Fig. 23 B). 
Each appendage contains an extension of the mesoderm and becomes 
differentiated into a series of parts, the limb segments, or podites, which 
are eventually movable on each other through the development of muscle 
fibers in the mesoderm of the appendage. Each body segment between 
the prostomium and the periproct potentially may develop a pair of 
appendages; but various appendages suppressed in the adult may not 
appear even in the embryo. The possession of jointed segmental append- 
ages separates the arthropods from the annelids and undoubtedly has 
given them the possibility of attaining the high degree of development 
that sets them so far above their wormlike relatives. 

Completion of the Body Wall. — ^The embrjm, we have seen, begins its 
development as a disc or band of thickened cells on the ventral side of the 
egg (Fig. 9, GB). The germinal area may be but a small part of the 
blastoderm, or it may almost encircle it, but there is always left a dorsal 
region {DBl) consisting of very thin attenuate cells that take no direct 
part in the formation of the definitive insect. As the marginal parts of 
the germ band increase in extent, they spread dorsallj’’ over the egg, and 
the extraembryonic field of the blastoderm contracts until it is con- 
densed into a cell mass that sinks into the yolk. The contracting dorsal 
blastoderm carries with it the ruptured amniotic folds, if the embrj*o 
has an amniotic covering, and all these tissues drawn into the j’olk are 
there absorbed to be utilized as nutriment bj' the growing embrj'o. 
With the disappearance of the dorsal blastoderm, the back of the embrjm 
is completed bj' the dorsal closure of the extended lateral plates, and the 
bodj' of the embryo thus becomes an OA'al or elongate sac with continuous 
walls formed from the ectodermal layer of the primitive germ band. 



38 


PRINCIPLES OF INSECT MORPHOLOGY 


Rep 
DMcl '■ 


Cdbl CdS Cdbl 



^Epd 


EMsd / ' _ , ... .. 

’.O'. .■■■%] 


IMsd 


The Definitive Body Cavity —In many adult annelids the body 
cavity consists of a series of segmental compartments which are the 
enlarged cavities of the coelomic sacs of the embryo. The arthropods 
have a continuous body cavity formed in part only from the rudimentary 
coelomic clefts in the embryonic mesoderm (Fig. 19 A, Cod). As the 
arthropod embryo develops, the median strand of the mesoderm breaks 

apart and some of its cells are said 
to be converted into free blood cells 
(Fig. 19 B, BCh). The space in 
•Ment which the blood cells lie is known 
as the epineural sinus {EpnS) ; it is 
a part of the haemocoele, or blood 
cavity of the insect, which is the 
remnant of the blastocoele after the 
invasion of the latter by the meso- 
,r!,^ .rL derm. The coelomic clefts become 

BCls VNC VMol . V -J r .V j 

Fig. 21. — Diagrammatic cross section COntUlUOUS HI 63,011 SldG 01 trllG DOCly 

of a nearly mature insect embryo. Apd, disintegration of the tranSVerse 

segmental appendage; BC, definitive body , , i • ii 

cai-ity; scis, blood cells; Cdbl, cardio- partitions, and theiT inner walls 
blasts; CdS, cardiac sinus; Z)il/cZ, rudiment qj. Jggg break doWU, The 

of dorsal muscles; EMsd, external meso- ... 

derm layer (somatopleure) ; Epd, epider- epmeural SmUS thus becomes COn- 
misj/ilfsd, inner mesoderm layer (splanch- the COelomic SpaceS. 

nopleure); Merit, mesenteron; Rep, ft-, j 

rudiment of reproductive organ; Tra, The cavity SO formed enlarges Upward 

trachea; YMcl, rudiment of ventral .^^,jth the growth of the meSoderm 
muscles; Five, ventral nerve cord; y, yolk. ° 

around the yolk or the alimentary 

canal, penetrates into the appendages, and forms eventually the 
continuous body cavity of the segmented body of the arthropod (Fig. 
21, BC). 



The mesoderm in each side of the body is split by the coelomic clefts 
into an outer lamina, or somatic layer (Fig. 21, EMsd), and into an inner 
lamina, or splanchnic layer (IMsd). The first becomes applied to the 
body waU, the second to the wall of the alimentary canal. 

The Mesodermal Organs. — ^From the mesoderm are formed the 
muscles, the heart, the blood cells, the fat body, connective tissue, and 
the parts of the reproductive organs that are not of ectodermal origin. 
Since the development of the principal mesodermal organs will be given 
in the chapters on the circulatory system, the fat body, and the repro- 
ductive system, we need give but brief attention to the subject here. 

The Muscles. — The muscle fibers of arthropods are all of the striated 
type of structure and are multinucleate, the nuclei in some cases being 
superficial and in others buried within the body of the fiber. Investi- 
gators differ as to whether each fiber represents a single multinucleate cell 
or is the product of many united cells. The muscles of the body wall and 



GENERAL ORGANIZATION AND DEVELOPMENT 


39 


the appendages and probablj'' the dilator or suspensory muscles of the 
alimentary canal are derived from the somatic layer of tlic mesodei-m; 
those that surround the digestive tube, including the ectodermal stomo- 
daeum and proctodaeum, are said to be all derived from the splanchnic 
layer. 

The Dorsal Blood Vessel. — The dorsal blood vessel, or principal cir- 
culatory organ of the insect, is a muscular tube Ijdng along the dorsal 
midline of the body. Its lumen is derived from the dorsal part of the 
haemocoele which formed a cardiac sinus (Fig. 21, CdS) enclosed between 
the upper undivided ends of the lateral mesoderm bands. As the latter, 
or cardidblasis (Cdbl), approach each other above the alimentary canal, 
their inner faces become hollowed as two opposing furrows; and when 


DV 



Fia. 22. — Cross section through the dorsal part of the abdomen of a male ombr.vo of tlio 
honey bee. (From Pclrunkccilch, 1903.) 

finally the two layers meet, the margins of their grooves unite to form a 
tube, which is the dorsal blood vessel (Fig. 22, DV). The walls of the 
vessel are converted into transverse muscle fibers, which give the tube 
a strong contractile power. The blood gains access to the heart cavity 
through lateral openings, oslia, in its walls; it is driven forward and 
expelled from the anterior end beneath the brain. 

The Fat Body. — The jirincipal fat-containing tissue of insects consti- 
tutes a definite structure known as the /a/ body, which consists of a loose 
aggregate of mesodermal cells lying in the body cavit 3 ^ The first cells 
of the fat body are formed in the embryo, but the organ increases greatly 
in size bj’’ cell multiplication and cell growth during j^ostcmbrj'onic 
stages, when the cells take on other functions besides that of fat storage. 

The Organs of Reproduction. — The intci’iial parts of the rejjroductive 
organs, in which the germ cells arc lodged, and in which these cells undergo 
their subsequent development into the spermatozoa or eggs, accordiiig to 
their sex, appear first as thickenings of the sjjlanchnic walls of the nie.so- 
derm in the abdominal region of the bod}'. The cell groups that form 
these thickenings, or genital ridges, arc the rudiments of the tc.‘!(cs or 
ovaries (Fig. 22, Rep). From each a strand of cells continues rearward, 
which becomes the duct of the organ, a vas deferens in the male, an 



40 PRINCIPLES OF INSECT MORPHOLOGY 

oviduct in the female. In a few of the lower insects each duct opens by 
an independent aperture near the posterior end of the body ; but in most 
insects the two vasa deferentia or the paired oviducts unite in a single 
median tube produced by an invagination of the ectoderm. The ecto- 
dermal tube thus formed becomes the ductus ejaculatorius in the male 
(Fig. 292 A, Dej), and the oviductus communis of the female (Fig. 284 A, 
Ode). 

The Tracheal System. — ^The respiratory organs of insects are entirely 
ectodermal; they consist both of evaginations and of invaginations of the 
body wall. The latter, taking the form of branching tubes, or tracheae, 
ramifying minutely throughout the body cavity, constitute the usual 
respiratory system of all insects whether aquatic or terrestrial. Respira- 
tory evaginations form “blood gills” and occur only in a few aquatic 
larvae. The rudiments of the tracheae appear first at a comparatively 
late stage of embryonic growth as segmental pits in the ectoderm along 
the sides of the body (Fig. 19 B, Tra). The pits deepen into tubes, the 
tubes fork out into branches, the branches subdivide until the tracheae 
from each primitive pit, the opening of which is now a spiracle, form a 
finely branched system in the corresponding half of the body segment. 
By the union of some of the branches from successive and opposite 
spiracles, longitudinal trunks and transverse commissures are established. 
Since the tracheal system is evidently a comparatively late acquisition 
in insects, it is not necessary to assume that all the segments once bore 
spiracles. There is embryonic evidence, however, of the former existence 
of spiracles on at least each of the segments from the last head segment to 
the tenth abdominal segment, as will be shown in Chap. XV. 

The Definitive Body Form. — ^The final external form of the insect 
may be traced through five theoretical evolutionary stages. 

First is the wormlike stage (Fig. 24 A) in which the animal consists of a 
long segmented part coextensive with the length of the alimentary canal 
and of a short unsegmented preoral part, or prostomium {Prst). The 
mouth {Mth) is situated ventrally between the prostomium and the first 
segment; the anus is terminal in the last segment, or periproct {Prpt). 
The prostomium contains the principal sensory ganglion and may be 
regarded as the primitive head, or archicephalon. 

In the second stage (Fig. 24 B) each body segment between the 
prostomium and the periproct acquires a pair of movable lateroventral 
appendages, and one or two pairs of antennal organs may be developed 
on the prostomium. 

The third stage (Fig. 24 C) is characterized by the union of the first 
postoral somite with the primitive head to form a composite head struc- 
ture, which, being the first stage in the evolution of the definitive head, 
may be termed the protocephalon (Prc). The somite involved is that 



GENERAL ORGANIZATION AND DEVELOPMENT 


41 


bearing the postantennal appendages, or second antennae (Fig. 23 B, 
Pnt), which in msects are reduced in the cmbrj'o and disappear in the 
adult, while the somite itself loses its identity after union with the 
procephalic lobes (C), though its ganglia become the tritoccrebral lobes 
of the brain. The protocephalon forms the procephalic part of the 
definitive insect head, but in some Crustacea it remains as a distinct 
cephalic structure bearing the e 5 '^es, the labrum, and the two pairs of 
antennae. The protocephalon, or the procephalic part of the definitive 
arthropod head, is commonly regarded as containing two primitive 
somites between the prostomium and the tritoccrebral segment, cor- 
responding to preantennal and the first antennal appendages, but there is 



Fig. 23. — Differentiation of tno sections (tOKmata) of the insect trunk ciurinK embryonic 
development. A, young embryo of Leptinolarsa. (From Whedrr, ISSO.) 15, cmI)iyo 
of Anurida witli appendage rudiments. (From Whrclcr, IS93.) C, embryo of Xaucorin, 
with trunk differentiated into protocephalic, gnathal, thoracic, and abdominal sections. 
(From Heymons, 1S99.) 

reason to believe that the antennal and preantennal regions of the head are 
prostomial in origin, and that the postantennal, or tritoccrebral, region 
represents the first primitive .somite, which is postoral. This view is 
e.xpressed in the diagrams (Fig. 24) and will be amplified in subsequent 
chapters on the head and the nervous sA'sfem. 

The fourth stage in the develoinncnt of the bodj' form (Fig. 24 D) 
differentiates the bisects from all other arthropods. It is well shown 
in the embrj'o of many insects (Fig. 23 C). In this stage the trunk seg- 
ments back of the protocephalon {Pre) become segregated into three 
regions. The first may be called the gnathal region (Gn), since its append- 
ages are destined to become feeding organs; the second is the thoracic 
region (Th), set apart as the locomotor center of the insect in' the special 
development of its appendages as locoinotoiy organs; the third is the 
abdominal region (Ab), on which the appendages are reduced and mostly 
obliterated. 



42 


PRINCIPLES OF INSECT MORPHOLOGY 


In tlio fifth stage (Fig. 24 E) the body of the insect attains its final 
form. The gnathal segments are now united with the protocephalon 
in the definitive head (H), which thus comprises, the prostomium and four 
succeeding segments and carries the gnathal appendages as well as the 
protocephalic appendages. In the Pterygota the thorax acquires wing 
rudiments in the form of paranotal lobes, two pairs of which eventually 



^ 1 eproauctive activities. — 

a by wMcl 

aside a few cells to nrocrp!+ group of cells' like itself, sets 

soma from the others we ^ ^ ^ species and then forms a highly organized 
systems of organs ^ «tudy of the Lions 

entity and fulfills its destinv maintains itself as a living 

Before the insect ettoee^/XST-r,' “> “• 

out of the egg shell. functions, however, it must get 



GENERAL ORGANIZATION AND DEVELOPMENT 


43 


Hatching. — A young animal enclosed in a hard-shelled egg is left 
to gain its liberty by its own exertions; with most insects maternal 
responsibility ends with the deposition of the eggs. Many insects at 
hatching, moreover, find themselves in a situation where it is yet impos- 
sible to begin their destined postembryonic life, for the eggs may be 
buried in the ground, contained in a chamber excavated in the wood of a 
tree by the mother, hidden in a deep crevice, covered with a protective 
substance, or enclosed in a horny case. Hence the young creature is 
often confronted with a hard task to be performed during the first few 
minutes or half hour of its active existence. Its escape from its first 
surroundings, however, is not made in a haphazard manner but is orderlj’' 
directed by “instincts,” and often the young insect is provided with 
special structures to lighten its task. 

The first adult function to become active in the hatching insect 
larva is tracheal respiration. The embryo within the egg makes use of 
the oxygen that diffuses through the egg shell and is dissolved in the 
body fluid; the entire tracheal system at this time is filled with a clear 
liquid. With the first movements of the young larva preparatory to 
hatching, however, the tracheal liquid is quickly replaced by free air, 
the liquid being drawn back from the main tracheal branches into the 
tracheoles, from which it finally disappears, while air simultaneously 
fills the tubes as the liquid retreats. So mysterious in aspect is this 
rapid aeration of the tracheal system that various impossible theories 
have been proposed to account for it on the assumption that the entering 
air drives out the liquid. It has been shown by Keilin (1924a), however, 
that the activating force must be the retraction of the liquid, and that the 
air automatically fills the emptied tracheae, since the walls of the latter 
are constructed to withstand atmospheric pressure. Sikes and Wiggles- 
worth (1931) point out that the mechanism that brings about the absorp- 
tion of the tracheal liquid at this time is probably the same as that which 
is functional after hatching, namely, that it is osmotic tension created in 
the blood by the discharge of metabolites resulting from the first muscular 
activities of the larva, which causes the tracheal liquid to be drawn into 
the blood or the tissues through the semipermeable walls of the tracheae, 
as in ordinary tracheole respiration (see page 459). If the larva has an 
open tracheal system, air can enter the tracheae through the spiracles, 
but if the latter are closed it must diffuse into the tracheae from the blood. 

The process of hatching, or eclosion from the egg, is accomplished by 
some insects simply by gnawing a hole through the chorion with the 
mandibles. With most insects, how^ever, the jaws are not free organs 
at the time of hatching, or they may not be of the biting type of structure. 
In such cases the insect must rupture the egg shell by body movements, 
and it olten gives itself for this purpose a closer contact against the chorion 



44 


PRINCIPLES OF INSECT MORPHOLOGY 


by swallowing the amniotic fluid, or air that has diffused through the 
chorion. The anterior end of the egg shell, or sometimes both ends, may 
be merely pushed off by the muscular exertions of the larva; but often the 
young insect is equipped with a special instrument, known as the “egg 
burster,” having the form of spines, a series of teeth, or a sharp ridge on 
the top of the head, with which a slit is made in the chorion over the head. 
Once the chorion is split, the insect, usually distending itself by copious 
draughts of air, issues rapidly from the cleft. 

All insects with incomplete metamorphosis and some holometabolous 
species are invested at the time of hatching in a thin cuticular sheath 
formed during embryonic development. The membrane has not been 
generally observed in holometabolous insects, but Smith (1922) has 
described it in three species of Neuroptera, and Sikes and Wigglesworth 
(1931) note its presence in the mealworm Tenebrio molitor. It seems 
probable, therefore, that the membrane occurs in other holometabolous 
insects, but that it is perhaps shed before hatching and is thus unobserved. 
A similar hatching membrane is of common occurrence in many other 
arthropods than insects. 

The hatching membrane may be a smooth, tightly stretched pellicle 
investing the body, or it may have extensions over the appendages, which 
are then cramped and folded in their pouches. The egg burster is a part 
of this embryonic investiture in most cases, though in the fleas it is said 
to be a strong ridge on the head of the definitive first-instar larva. Some 
insects shed the embryonic cuticula at the time of hatching and leave the 
shriveled membrane in the egg; others reject it just after emergence; 
but young grasshoppers retain it until they have burrowed upward to the 
surface of the ground from their subterranean egg pods, and young cicadas 
work their way out of the egg pockets in the twigs of trees still bound in 
their hatching vestments, which are discarded at the mouth of the cavity. 


GLOSSARY OF EMBRYOLOGICAL TERMS 


Amnion (Am). The cellular, membranous covering of the embryo formed from 
folds of the extraembryonic blastoderm; or, specifically, the inner layer of each 
amniotic fold, or of the completed covering. 

Amniotic Cavity (AmC). — The cavity between the amnion and the embryo. 

Anterior Mesenteron Rudiment (AMR). — The anterior group of cells of the 
ventral endoderm remnant that regenerates the mesenteron. 


Archenteron (Gc). — The gastrocoele, or cavity of the endoderm. 

Archicephalon (Prst).— The primitive annehd-arthropod head, or prostomium. 
Aichicerebrum (Arc).— The primitive suprastomodaeal nerve mass of the pro- 
stomium. (Archencephalon.) 

Blastocoele (Be).— The cavity of the blastula. {Segmentaiion cavity.) 

Blastoderm {Bid).— The surface layer of cells of the blastula before gastrulation. 
Blastomeres.— The cleavage cells, or cells produced by the division of the egg or 
its nucleus that form the blastoderm. 



GENERAL ORGANIZATION AND DEVELOPMENT 


45 


Blastopore {Bpr). — The mouth of the gastrulation cavity. 

Blastula. The early stage of the embryo in which the only cell layer is the blasto- 
derm. 

Body Cavily (BC). — The definitive cavity of the body and appendages, not strictly 
equivalent in all animals. 

Cardiac Sinus (CdS). — The channel of the haemocoele dorsal to the yolk or ali- 
mentary canal. 

Cardioblasts (Cdbl). — The dorsal strands of mesodermal cells that form the dorsal 
blood vessel. 

Cephalic Lobes (Prc). — ^The head lobes of the embryo, comprising the region of 
the prostomium and usually that of the tritocerebral somite. 

Chorion (Cho). — ^The nonchitinous shell of an insect egg, formed in the egg follicle. 

Cleavage. — The division of the egg or its nucleus and of the resulting cells forming 
the blastoderm. {Segmentation of the egg.) 

Cleavage Cells {CCls). — The cells formed during cleavage. 

Coelome. — A body cavity formed of the coelomic sacs only. 

Coelomic Sacs (Coel). — The paired segmental cavities of the mesoderm. {Primi- 
tive segments, Ursegmente.) 

Cortical Cytoplasm {cpVj. — The peripheral layer of cytoplasm in the egg. {Keirtv- 
hauthlastem.) 

Deutoplasm (F). — The yolk, or nutritive materials of the egg enmeshed in the 
cytoplasm. (See yolk.) 

Dorsal Blastoderm {DBl). — The extraembryonic part of the blastoderm. {Serosa.) 

Dorsal Organ {DO). — A mass of cells in the dorsal part of the embryo apparently 
produced by the invaginated serosa. 

Ectoderm {Ecd). — The outer cell layer of the embryo. 

Endoderm {End). — The innermost cell layer of the embryo, forming the epithelium 
of the stomach. 

Epineural Sinus {EpnS). — The channel of the embr5mnic haemocoele beneath the 
yolk or alimentary canal. 

Gastrocoele {Go). — The gastrulation cavity. {Archenteron.) 

Gastrula. — The embryo after gastrulation. 

Gastrulation. — The formation of the endoderm, either by invagination of one 
wall of the blastula or by internal proliferation of cells from the blastoderm. 

Germ Band {GB). — The area of thickened cells on the ventral side of the blasto- 
derm that becomes the embryo. {Embryonic rudiment, germ disc, primitive streak. 
Keimstreif, Keimscheibe, bandelelte primitive, plaque ventrale, piastra germinativa.) 

Germ Cells {GCls). — Cells destined to become ova or spermatozoa, difierentiated 
from the somatic cells during cleavage. 

Germ Tract. — The cytoplasmic area of the blastula containing the germ cells. 
{Posterior polar plasm, Keimbahn.) 

Gnathal Segments {Gn). — The segments of the insect embryo the appendages of 
which become the mandibles and first and second maxillae. 

Haemocoele. — The blood cavity or cavities of the embryo between the mesoderm 
and the other germ layers, probably a remnant of the blastocoele. 

Hatching Membrane. — A membranous sheath investing the young insect at the 
time of hatching, probably an embryonic exuvial cuticula, shed during hatching or 
shortly after. (Not the amnion.) 

Holoblastic Division. — ^The type of cleavage in which the entire egg is divided. 

Intermediate Mesenteron Rudiment {IMR). — A median strand of cells of the 
ventral endoderm remnant taking part in the regeneration of the mesenteron in some 
insects. 



46 


PRINCIPLES OF INSECT MORPHOLOGY 


Invagination of the Embryo. — The direct infolding of the embryo into the egg. 

Involution of the Embryo. — Invagination of the embryo accompanied by a revolu- 
tion and final reversal of position in the egg. 

Lateral Nerve Cords (LC). — The lateral strands of nerve tissue produced from the 
ventral neuroblasts. 

Lateral Plates (LP).— The lateral areas of the germ band after differentiation of 
the middle plate. 

Median Nerve Cord (ilfC). — The median strand of nerve tissue produced from 
the ventral neuroblasts. 

Meroblastic Division. — The type of egg cleavage in which only the nucleus and 
the nuclear cytoplasm are divided. 

Mesenchyme. — Mesoblastic tissue formed of loosely conneeted or scattered cells. 

Mesenteron (Merit). — The stomach of the embryo; in insects regenerated from 
scattered endodermal cells or from intact endodermal remnants; becomes the epi- 
thelium of the adult stomach, or ventriculus. 

Mesenteron Rudiments. — The groups of endoderm cells that regenerate the 
mesenteron, including an anterior, a posterior, and sometimes an intermediate 
rudiment. 

Mesoblast. — The middle cell tissue of the embryo, including mesenchyme and 
mesoderm. 

Mesoderm (Msd). — The mesoblastic tissue that takes the form of a definite mid- 
dle cell layer. 

Metamere. — One of the body divisions of the embryo; an embrjmnic somite or 
primary body segment. 

Micropyle. — The pore or group of pores in the egg chorion giving entrance to the 
spermatozoa. 

Middle Plate (MP). — The median strip of cells in the germ band between the 
lateral plates. 

Morula. — The mass of cells formed by holoblastic cleavage of the egg. 

Neural Groove (NIG ). — The median ventral groove of the embryo between the 
neural ridges. 

Neural Ridges (NIR ). — The two longitudinal ventral ridges of the embryo in 
which are formed the lateral cords of neuroblasts. 

Neuroblasts (Nbl ). — The primitive nerve cells differentiated from the ectoderm. 

Nuclear Cytoplasm (npl). — The small mass of egg cytoplasm containing the egg 
nucleus. 

Ootheca. An egg case formed of secretion products of accessory genital glands 
or the oviducts. 

Ovum. — The mature female germ cell. 

Periproct (Prpt). The terminal piece of the body containing the anus, anterior to 
which segmentation takes place. {Telson.) 

Posterior Mesenteron Rudiment {PMR). — The posterior group of cells of the 
ventral endoderm remnant that regenerates the mesenteron. 

Proctodaeum (Proc). The posterior ectodermal part of the alimentary canal. 

Prostomium (Prst). — The anterior preoral unsegmented part of the trunk of a 
segmented animal. (Acron.) 

Protocephalon (Frc). A general early stage in the evolution of the arthropod 
ead, corresponding to the cephalic lobes of the embryo, comprising the prostomium 
and usually the first postoral somite, forming the procephalic region of the definitive 
msect head. 

Segment.— A subdivision of the body or of an appendage between areas of flexi- 
bility associated with muscle attachments. A primitive body segment is a somite: 
a segment of an appendage is a vodite. An embryonic body segment is a metamere. 



GENERAL ORGANIZATION AND DEVELOPMENT 


47 


Segmental Appendages. — The paired ventrolateral segmental outgrowths of the 
body wall serving primarily for locomotion. 

Serosa (<S'er). — The dorsal blastoderm or its extension as the outer layer of the 
amniotic covering of the embryo. 

Soma. — The body of an animal as distinguished from the germ cells. 

Somatic Cells. — The body cells as distinguished from the germ cells. 

Somatic Layer (EMsd). — The external layer of the mesoderm applied against the 
body wall. {Somatopleure.) 

Somite. — A primary body segment, or metamere. 

Spermatozoon. — A mature male germ cell. 

Splanchnic Layer (IMsd). — The inner layer of the mesoderm applied to the walls 
of the alimentary canal. {Splanchnopleure.) 

Stomodaeum {Stom). — The anterior ectodermal part of the alimentary canal. 

Syncephalon. — A secondary composite head, formed of the prostomium and one or 
more succeeding somites. 

Ventriculus {Vent). — The stomach of the adult insect, the epithelial wall of which 
is the endodermal mesenteron of the embryo. 

Vitelline Membrane {Vit). — The wall of the egg cell; undivided in meroblastic 
cleavage, surrounding the blastoderm. 

Vitellophags {Vph). — Endoderm cells proliferated into the yolk and accomplishing 
a partial digestion of the latter. 

Yolk (F). — Deutoplasm. When the yolk is small in quantity or evenly distributed 
through the cytoplasm, the egg is said to be alecithal or hololecithal; when the yolk has 
a central position, the egg is centrolecithal; if the yolk hes at one end of the egg, the 
latter is telolecilhal. Insect eggs are centrolecithal. 

Yolk Cells (YCls). — Cleavage cells remaining in the yolk and taking no part in the 
blastoderm formation. 

Yolk Cleavage. — The division of the yolk into masses containing from one to 
several cleavage nuclei. 

Zygote. — The fertilized egg or egg nucleus. 



CHAPTER III 

THE BODY WALL AND ITS DERIVATIVES 

The body wall of an animal is that part of the ectoderm which remains 
at the surface in the fully developed stage and serves to maintain anatom- 
ical integrity in the rest of the organism. Though primarily an integu- 
ment, because of its position numerous responsibilities devolve upon the 
body wall: it must bear the brunt of all external things and forms of 
energy that touch upon the animal; it must be able to receive impressions 
of changes in the environment to which it is advantageous or necessary 
that the creature should respond; and in the arthropods it is the principal 
agent of the motor mechanism. 

In following the development of the arthropod embryo we have seen 
how one group of cells after another becomes inflected from the ectoderm 
and specialized to form some internal organ or group of organs. The 
body wall, therefore, is an undifferentiated remnant of the ectoderm, and 
as a consequence its cells preserve in a high degree the potentiality of 
primitive functions, which may be expressed in the adult as absorption, 
transpiration, secretion, excretion, and sensitivity, while, furthermore, 
they retain a large capacity for development, as shown in the many 
specialized organs that have been evolved in the integument. In a study 
of the insect body wall, therefore, while we give chief attention to its 
fundamental structure and to the modifications adaptive to protection 
and locomotion, we must recognize that the various ectodermal glands 
and the sense organs are directly parts of it, and that, more remotely, 
the invaginations forming the anterior and posterior sections of the 
alimentary canal, the respiratory organs, the unpaired reproductive 
ducts, and the entire nervous system are derivatives from it. 

1. THE BODY WALL 

The arthropod body wall is reinforced by a cuticula covering its outer 
surface. The cuticuiar substances are products of the body-wall cells, 
but the protective layer formed by them becomes the most important 
part of the integument, and the matrix cells, after having generated the 
cuticula, take a subordinate place in the tegumentary structure. The 
cuticula may be entirely soft and flexible, but characteristically it becomes 
sclerotized, or hardened in certain areas, forming body-wall plates, or 
sderites. The sclerites, which usually have definite shapes and interrela- 

48 



THE BODY WALL AND ITS DERIVATIVES 


49 


tions, constitute the exoskeleton of the arthropod and play an important 
role in the motor mechanisms of the animal. Inflections of the body 
wall containmg rigid ingrowths of the cuticula, or apodemes, form collec- 
tively the endosheleton of the arthropod organization. The flexible lines 
of the integument between sclerites are usually called sutures, and movable 
points of contact are termed articulations. The hard exoskeleton pro- 
duces a mechanical and physiological condition in the arthropods quite 
different from that of soft-skinned animals, and Kennedy (1927) in an 
interesting paper has pointed out many ways in which the exoskeleton 
has been a factor both in limiting and in directing the evolution of insects. 

Structure of the Body Wall. — ^The arthropod integument, because of 
the presence of the cuticula, has a stratified structure (Fig. 25 A), since it 
consists of both the inner cellular epithelium {Epd) and the outer non- 
cellular cuticula {Ct). In entomology the cell layer of the body wall is 



Fig. 25. — Structure of the body wall, diagrammatic. A, piece of body wall bearing 
a movable external process, or seta (a), and an immovable process (b). B, vertical section 
of body wall. BMh, basement membrane; Ct, cuticula; En<A, endocuticula; Epdt, 
epicuticula; Fpd, epidermis; Excl, exocuticula. 

commonly called the “hypodermis,” but the term epidermis is preferable 
inasmuch as the integumental epithelium is the homologue of the ecto- 
dermal layer of the skin so designated in vertebrate anatomy, though 
either term is illogical when applied to invertebrates having no accom- 
panying “dermis.” Internally the epidermis is limited by a very thin 
noncellular basement membrane (BMb). 

The Cuticula. — The cuticula itself generally has a stratified appearance 
in sections, since it usually exhibits two distinct principal layers, namely, 
an outer primary cuticula, or exocuticula (Fig. 25 B, Exct), and an inner 
secondary cuticula, or endocuticula (Encf), while on the exterior there is a 
very thin surface layer, or epicuticula (Epci), which appears in sections as 
a clear border line about one micron in thickness. The characteristic 
constituent of the exocuticula and endocuticula is chitin, but the exocu- 
ticula contains also other substances and is generally distinguishable from 
the endocuticula by its darker pigmentation and its denser structure, 
since it is the layer of the body wall containing the hardening substances 
that form the sclerites. The epicuticula is a nonchitinous layer, but its 


50 


PRINCIPLES OF INSECT MORPHOLOGY 


component materials also permeate the exocuticula (Wigglesworth, 
19336). 

The cuticular substance known as chitin is a colorless nitrogenous 
polysaccharide, perhaps of microcrystalline structure (Gonell, 1926). 
Chemically, as stated by W. J. Schmidt (1930), “chitin is characterized 
by its decomposition product, glucosamine, as an animopolysaccharide, 
one of those interesting compounds intermediate between carbohydrates 
and proteins.” Its chemical formula as given by Brach (1912) is 
(C32Hb4N402i)x. As stated by Campbell (1929), chitin is insoluble in 
water, alcohol, ether, dilute acids, and dilute or concentrated alkalies. 
It is dissolved, with or without decomposition, by concentrated mineral 
acids and, according to Schulze and Kunike, by water-free formic acid. 
It is hydrolyzed by concentrated mineral acids, with the formation of a 
glucosamine salt or chitose, a sugar, and fatty acids, chiefly acetic. 
Treatment of chitin with potassium or sodium hydroxide at high tempera- 
tures also hydrolyzes it, producing chitosan and acetic acid, but without 
change in appearance. Chitin is oxidized and dissolved at room tempera- 
ture by a solution of sodium hypochlorite containing 5 per cent of avail- 
able chlorine. It is not attacked by mammalian digestive enzymes but 
is broken down by Bacillus chiiinovorus (Benecke, 1905), which may be 
the agent of its decomposition in nature. 

Rdntgenographic tests and polarized light studies, as described by 
W. J. Schmidt (1930), have shown that chitin has a fibrous structure, and 
that the fibers are composed of elongate, submicroscopic, crystalline parts 
(micellae) which lie parallel with the fiber axes. The chitinous mass, 
furthermore, is penetrated by fine intermicellar spaces and therefore 
possesses submicroscopic pores. This porous character, Schmidt points 
out, accounts for the permeability of chitin to gases and liquids, as in 
the chitin-covered chemoreceptive sense organs, tracheae, absorptive 
surfaces of the alimentary canal, and discharging surfaces of glands. 

The common laboratory practice of soaking or boiling parts of the 
insect body wall in caustic solutions to soften and clear the cuticula 
removes the coloring and hardening substances from the latter and may 
change the chitin into chitosan ; but it does not disintegrate the cuticula 
or produce any visible change in its chitinous parts because chitosan does 
not differ in appearance from chitin. The insect cuticula can be stained 
with acid fuchsin after soaking in potassium hydroxide until translucent 
and then washing thoroughly, finally in acidulated water. 

For determining the presence of chitin, Campbell (1929) gives 
the following practical test, a modification of the more difficult Van 
Wisselingh-Brunswick method. Material suspected of being chitinous 
is placed in potassium hydroxide solution saturated at room temperature 
and kept at 160 C. for 15 minutes in a tube closed by a Bunsen valve. 



THE BODY WALL AND ITS DERIVATIVES 51 

Since chitosan gives a violet color reaction with solutions of iodine in 
weak acids, this test may now be applied to a sample on a slide, after 
which the material should be dissolved on the slide at room temperature in 
a drop of 75 per cent (by volume) sulphuric acid. This gives chitosan 
sulphate, crystals of which, precipitated by slow dilution of the drop in 
moist air, cling to the slide, where they may be washed, stained with an 
acid dye, and mounted in balsam. 

Chitin is of wide distribution among invertebrate animals, being found, 
according to Wester (1910), in the Porifera, Hydrozoa, Bryozoa, Brachio- 
poda, MoUusca, Annelida, and Arthropoda. It is unknown in the 
Protozoa and Vertebrata; among plants it is restricted to the fungi, in 
which group it was first discovered. Chitin is perhaps the organic 
foundation of the cuticula of all arthropods ; it occurs also in the intima of 
the various ectodermal invaginations and has been reported to be present 
in the lining (peritrophic membrane) of the mesenteron in several species 
of insects. In the Aimelida chitin forms the bristles or setae of the skin 
but it is not present in the cuticula. In the Onychophora, however, the 
integument is said to be chitinous. 

It is a common mistake to suppose that the sclerites of the insect 
body wall are "strongly chitinized” areas of the cuticula. The reverse 
probably is more generally true, since sclerotization results from the 
deposit of nonchitinous substances in the exocuticula. Campbell (1929) 
has shown that the exocuticula of Periplaneta contains only about 22 per 
cent of chitin, while the soft endocuticula contains about 60 per cent, and 
Kunike (1926) found that the wing covers of a May beetle contain by 
weight 75 per cent of nonchitinous substances, and those of a grasshopper 
as much as 80 per cent. The chemical nature of the hardening substances 
that form the sclerites of the insect cuticula is not knovm, though there is 
some evidence of their being carbohydrates. In the Crustacea the 
sclerotic matter of the integument is largely calcium salts. 

The nonchitinous epicuticula is composed of substances that, it 
has been shown by Wigglesworth (19336), are also constituents of the 
exocuticula. According to Kiihnelt (1928, 1928a), this surface film 
(Grenzlamelle) of the exocuticula is highly resistant to acids, but when 
heated in caustics it is saponified and can be shovTi to contain fatty 
acids and cholesterin. From its chemical reactions the epicuticula of the 
insect body wall appears to be closely related to the surface cuticula of 
plants. As Kiihnelt points out, it protects the insect against many harm- 
ful external influences, such as excessive humidity, dryness, and disease 
organisms, and makes it possible for insects to live under a great variety 
of environmental conditions. 

The histological appearance of the cuticula varies somewhat in 
different insects and in different parts of the integument of the same 



52 PRINCIPLES OF INSECT MORPHOLOGY 

insect. Most investigators find that the endocuticula has a faint 
horizontally lamellate structure, in which usually there are visible 
fine vertical striations. The striations appear to be canals left by 
protoplasmic filaments that, during the formative stage of the cuticula, 
extend outward from the epidermal cells. The cuticular material is 
probably laid down in layers between these filaments, which are later 
retracted. N. Holmgren (1902) has suggested that the protoplasmic 
strands of the epidermis represent primitive cilia that once may have 
covered the bodies of the arthropod ancestors. 

In some of the Coleoptera the cuticula has a highly specialized 
structure. The cuticula of the larva of Dytiscus or Lucanus, according 
to Kapzov (1911), is composed of stratified lamellae having a distinctly 
spongy or alveolate structure. In the endocuticula coarsely and finely 
alveolated lamellae alternate, the plates becoming thinner toward the 
periphery, while in the exocuticula the lamellae are condensed into a 
more compact fabric in which the alveolation is almost obliterated. In 
the adult beetles, as shown by the studies of Biedermann (1903) and 
Kapzov (1911) on Lucanus cervus, and of Casper (1913; Korschelt, 1924) 
on Dytiscus, the cuticula has a much more complicated structure. The 
exocuticula is a simple alveolar tissue showing no stratification or stria- 
tion. The endocuticula, on the other hand, consists of horizontal 
bars, or trabeculae, arranged in well-defined strata. The trabeculae lie 
parallel in each stratum, but those of successive strata are crossed at 
definite angles. In Lucanus the trabeculae are bound together by 
bundles of fibrous strands that extend between them from the epidermal 
cells; in Dytiscus, according to Casper, the uniting strands are visible 
only in an early formative stage. The strands would appear to represent 
the protoplasmic processes of the epidermal cells observed in other insects. 

The Epidermis. — The ectodermal cells of the body wall are primarily 
arranged in a single layer (Fig. 25, Epd), and in most places they preserve 
the form of a simple epithelium. Secondarily they may become separated 
into two layers or disposed irregularly; in most such cases, however, each 
cell maintains its attachments both to the cuticula and to the basement 
membrane, though the connection with one or the other is reduced to a 
fine strand of protoplasmic tissue. In the growing stages of insects 
the epidermal cells are usually cubical or columnar, with the nuclei near 
their bases; but in adult insects, after the activity of cuticula formation 
is over, the matrix cells become more or less degenerate and appear in 
most places as a thin protoplasmic layer beneath the cuticula, in which 
cell boundaries are indistinct and the cell areas are marked only by the 
nuclei. 

The Basement Membrane.— The thin membrane that forms the inner 
lining of the body wall (Fig. 25, BMb) is so closely adherent to the 



THE BODY WALL AND ITS DERIVATIVES 


53 


epidermis that it appears to be a product of the latter similar to the 
cuticula covering the outer surface. It has often been stated that 
this basement membrane is a connective tissue layer, itself composed 
of greatly flattened and attenuated cells, but in sections of the body 
wall of insects we may look in vain for positive evidence of cellular 
structure in the basement membrane. Though the membrane may often 
appear at first sight to be nucleated, a closer inspection will show that 
the visible nuclei belong to blood ceUs or to a sheet of connective tissue 
that is here and there applied against the basement membrane. 

Sclerotization. — ^From the standpoint of morphology the most 
important feature of the arthropod body wall is its ability to produce 
definitely limited sclerotic areas in the cuticula. It is probable that 
sclerotization served first as a means of protection; but the presence 
of integumental plates in animals having the muscles attached on the 
body wall gave at once the possibility of a new mechanism of movement 
and of locomotion; and the development of refined interrelations between 
muscles and sclerites has been the principal line of evolution in the 
arthropods. The study of insects has been largely a study of sclerites. 
And yet we know practically nothing of the chemical or physical processes 
of cuticular sclerotization in insects. It is obvious, however, that only 
when we have learned something of the physiological nature of sclerites 
can we give them their proper status in comparative anatomy; and only 
on such a basis can we intelligently judge the value of sclerites as taxo- 
nomic characters. 

Sutures. — The term “suture” comes from the Latin word suere, 
“to sew.” In anatomy it properly applies, therefore, to the lines along 
which adjoining plates have united, as those between the centers of 
ossification in the vertebrate cranium. In entomology, however, almost 
any kind of line or narrow space separating sclerotic areas of the cuticula 
is called a suture. We may distinguish at least four distinct varieties of 
entomological sutures: (1) external grooves of linear inflections of the 
cuticula that form internal ridges or plates to strengthen the skeletal 
walls or to furnish increased surfaces for muscle attachments; (2) lines 
where the sclerotization of the cuticula has become secondarily dis- 
continuous in order to give flexibility; (3) lines where sclerotization has 
never taken place ; and (4) true sutures or lines of union between originally 
distinct sclerites. Perhaps the majority of insect sutures belong to the 
first category. In descriptive works they are recorded as lines delimiting 
subdivisions in a larger sclerotized field. It should be recognized, 
however, that in most such cases the “sclerite” so defined is merel}’’ an 
incidental result of the cuticular infoldings, and that the true functional 
characters are the endoskeletal structures formed by the inflections 
of the body wall, the lines of which appear e.xternally as “sutures.” 



54 


PRINCIPLES OF INSECT MORPHOLOGY 


Apodemes. — Any internal cuticular process of the body wall is an 
apodeme (from dro, “away,” and Setiaff, “body”). The cuticular part 
of an apodeme is always contained in a matrix of the epidermis (Fig. 
26 C, Epd) and is usually an inflection of the cuticula (A, B, C), though 
in some cases it appears to be a solid cuticular ingrowth (D, E). The 
site of a hollow apodeme is marked externally by a depressed line (suture) 
or a pit, according to the form of the apodemal invagination, and at 
ecdysis the cuticular core is withdrawn and regenerated. Apodemes 
ordinarily take the form of ridges, plates, or arms formed in multi- 
cellular invaginations of the epidermis (A, B, C), and they have definite 
mechanical purposes. Most of them are ridges which evidently serve 



Fig. 26. — Apodemes, or internal processes of the body wall. A, B, C, various forms 
of multicellular apodemes, diagrammatic. D, a unicellular apodeme. E, unicellular 
muscle “tendons” at the end of a multicellular apodeme. F, formative stage of the same. 
(E, F from Janet, 1907.) 


to strengthen the exoskeleton, either forming a brace between two points 
of stress or giving rigidity to an area subject to special strain. Armlike 
apodemes, or apophyses, usually furnish attachment points for muscles, 
though some muscle-bearing apodemes have the form of ridges or plates. 
Individual muscles are frequently attached to the body wall by long, 
slender, threadlike apodemes, often called “tendons,” which may have 
an expansion at the end for the reception of the muscle. In some cases 


very slender, tendonlike muscle apodemes are extensions of the cuticula 
formed within a single cell of the epidermis (E, F). These unicellular 
tendons give attachment each to a single muscle fiber. 

Articulations. Wherever there is a line of movement in the body 
wall, the flexible area, or “joint,” is merely the nonsclerotized cuticula 
between two neighboring regions of sclerotization (Fig. 27 A, mb). The 
movable area is known as an articular membrane, or corium. 

The movements possible at a joint will depend on the extent of the 
articular membrane; if the latter is ample and completely separates the 



THE BODY WALL AND ITS DERIVATIVES 


55 


sclerotic parts, as between the segments of the abdomen (Fig. 27B, jlfb), 
the movement is unrestricted. In most cases, however, particular!}'’ at 
the joints of the appendages, movement is limited by the special develop- 
ment of one or two pairs of contiguous points on the adjacent ends of 
the adjoining segments. Limited joints of this nature may be dis- 
tinguished as articulated joints, since the points of contact constitute 
specific articulations. According to whether an articulated joint has 
one pair or two pairs of articulating surfaces, it is said to be mono- 
condylic or dicondylic. A monocondylic joint may have a partial rotary 
movement; a dicondylic joint is restricted to a hinge movement. 

Articulations are of two types of structure; in one the points mak- 
ing contact are sclerotic prolongations within the articular membrane 
(Fig. 27 C, e, /) , in the other the articulating surfaces are areas of contact 



Fig. 27. — Sutures, joints, and articulations, diagrammatic. A, section through a 
simple membranous “suture.” B. a conjunctival membrane (.Mb) between two body 
segments. C, a dicondylic leg joint with intrinsic articulations (e, /). D, the typical 
extrinsic dicondylic articulation of the mandible with the cranium. 

on the outside of the skeletal parts (D, a, c). The two forms of articula- 
tions may be distinguished as intrinsic and extrinsic, respectively. 
Extrinsic articulations are usually of the ball-and-socket type and are 
particularly characteristic of the articulations of the mouth appendages 
vdth the wall of the cranium (D). The articulations of the legs with the 
body (the pleuro-coxal articulations) are intrinsic, as are usually also 
the articulations between the leg segments (C), though monocondylic 
leg articulations may be extrinsic. 

2. EXTERNAL PROCESSES OF THE BODY WALL 

The outer surface of the cuticula is seldom smooth or bare; it presents 
a great variety of microscopic roughenings in the form of points, pits, 
ridges, and sculptured designs, and it is covered with larger outgrowtlis 
that take the shape of spicules, spines, hairs, and scales. All the e.xternal 
processes of the body wall, however, may be classed in two groups 



56 


PRINCIPLES OF INSECT MORPHOLOGY 


according to whether the epidermal cells take a direct part in tlreir 
production or do not; that is, they are either cellular or noncellular 
outgrowths. Of the cellular processes, some are unicellular, others are 
multicellular. 

Noncellular Processes of the Body Wall.— The noncellular projec- 
tions of the outer surface of the body wall are purely cuticular structures 
(Fig. 28 A, B). They have the form of minute points or nodules (scobi- 
nations), spicules, small spines, hairs, corrugations, and ridges, the last 
often enclosing regular polygonal areas. The pattern of these surface 



Fig. 28. — External processes of the body wall, diagrammatic. A, B, noncellular 
cuticular processes. C, D, multicellular processes. E, a typical unicellular process, or 
seta. Alv, setal socket, or alveolus; Set, seta; smb, setal membrane; Tmg, tormogen, or 
socket-forming cell; Trg, trichogen, or seta-forming cell. 

characters appears in some cases to have a relation to that of the under- 
lying epidermal cells, but in others it seems to be entirely unrelated 
to the cell arrangement. These surface structures of the mature body 
wall are probably formed over cytoplasmic processes of the epidermis 
when the outer layers of the cuticula are being generated, and later 
become solid. 

Multicellular Processes of the Body Wall. — Cuticular structures 
of this nature are hollow outgrowths of the entire body wall and are 
therefore lined by a layer of formative epidermal cells (Fig. 28 C). They 
are usually large and spine-like in form. Most of them are solidly fixed 
to the surrounding cuticula (C), but some are set in a membranous ring 
and are movable (D). The immovable varieties are specifically termed 
spines, the movable ones are distinguished as spurs. Examples of fixed 
multicellular processes are seen in the spines of the hind tibiae of certain 
Orthoptera, while the spurs at the ends of the tibiae are examples of the 
movable variety. The lateral claws of the feet of insects are large 
movable sp’trs. Both spines and spurs may themselves bear unicellular 
processes, or setae (D). 



THE BODY WALL AND ITS DERIVATIVES 


57 



TTnicellular Processes of the Body Wall. — The typical outgrowths of 
the body wall in this class are the hairlike processes, termed setae, that 
constitute the principal body covering 
of most insects. Some unicellular 
processes, however, are thick and 
spinous, such being distinguished as 
spine-like setae; others are branched 
or featherlike and are termed plu- 
mose hairs; still others are flat squa- 
mous structures of various shapes, 
known as scales. Also there are uni- 
cellular outgrowths of many other 
varieties having the form of cones, 
pegs, hooks, spatulae, knobbed hairs, 
etc., but all are fundamentally setal 
structures. 

Structure of a Seta. — A typical seta 

is a slender hairlike process of the Fig. 29.— Development and structure 

cuticula formed by a plasmatic out- lepidoptorous wing scales. {From 
« .17 * 7 7 MaycTy 1896 .) A, early stage of scale 

growth from a single large epidermal 
cell (Fig. 28 E). In the mature con- 
dition the plasmatic core usually 
shrinks and more or less withdraws 
from the cavity of the seta, but at each 
moult the seta may be re-formed by a new outgrowth from the generative 
cell. The base of the seta is set in a small membranous ring of the body 
wall, known as the setal membrane (smb), which may be depressed in a 
hair socket, or setal alveolus (Alv), and the latter may be elevated on a 
tubercle. Beneath the base of the seta a cylindrical internal cavity of the 
cuticula, called the trichopore, contains the distal parts of the cells 
associated with the seta. 

The epidermal cell that forms a seta, or any hairlike structure, is 
termed the trichogenous cell, or trichogen (Fig. 28 E, Trg). Closely 
associated with the trichogen there is usually a second cell that forms 
the setal membrane, and which for this reason is sometimes called the 
membrane cell, but since the setal membrane is usually the floor of an 
alveolus its generative cell has been named bj'^ Wigglesworth (19335) the 
socket-forming cell, or torniogen (Tmg). The seta-forming process of 
the trichogen, during the period of the setal growth, is said to penetrate 
the tormogen like a finger thrust through a ring (Haffer, 1921; Wiggles- 
worth, 19336). In the mature condition, therefore, the distal part of 
the socket cell surrounds the neck of the hair cell, and the seta rises from 
the center of the setal membrane. Finallj', there is associated with 


cells (Sg) in section of a wing of Faucsso 
antiope. B, later stage of scale growth 
on pupal wing of Danats plexxppus. C, o 
mature wing scale of D. plexippus, D. 
cross section of mature scale of D. 
archippus. 



58 


PRINCIPLES OF INSECT MORPHOLOGY 


many insect setae, if not the majority of them, a sensory nerve cell, lying 
in or just beneath the epidermis, that is connected with the seta by a distal 
nerve process. Setae thus innervated become setal sense organs. 

Scales.—ThQ small, flat, scale-like structures that constitute the body 
covering of adult Lepidoptera and of some other insects are greatly modi- 
fied unicellular outgrowths of the body wall, which probably have been 
evolved from ordinary setae. Several stages in the development of a scale 
from a single cell of the wing epidermis of butterflies are shown in 
Fig. 29. Each scale arises as a blunt process (A, Sq) formed by an 
outgrowth from a special scale cell (Trg) of the wing epidermis. As the 
process elongates (B), it takes on the shape of a small bag and finally 
flattens out to form the scale (C). When the scale nears completion, 
the scale-forming cell degenerates and withdraws from the lumen of 
the scale. The two horizontal walls of the scale are united by vertical 
cuticular bars (D) , which serve to give rigidity to the scale by binding its 
upper and lower surfaces together. The pigmentation of the scale is 
said to be formed by blood corpuscles that enter the fully formed scale 
after the retraction of the primary scale cell. The iridescent colors so 
characteristic of most insect scales are the result of the surface sculpturing 
of the scale itself. 

Poison Setae . — The larvae of certain Lepidoptera are provided with 
setae from which is discharged an irritant venom formed in special 
poison gland cells associated with the trichogenous cell (Fig. 30 A). 
The poison issues from the ends of the setae when the tips of the latter 
(a) are broken off. A concise and comprehensive account of the poison 
apparatus of North American caterpillars is given in a paper by Gilmer 
(1925), to which the student is referred also for references to the work of 
other investigators on this subject. Species of caterpillars known to be 
poisonous occur in the Notodontidae, Liparidae, Megalopygidae, Arc- 
tiidae, Noctuidae, Eucleidae, Saturniidae, and Nymphalidae. 

According to Gilmer, there are two principal types of stinging struc- 
tures in caterpillars. In one type (Fig. 30 A) the apparatus consists of 
an ordinary seta (Set) that has become toxic by the development of a 
poison gland cell (GICT) immediately adjacent to the trichogen (Trg), 
which extends into the lumen of the seta. In the other t 3 ^e (B) the 
stinging elements are the same as in the first, but the seta has been 
carried out on the end of a multicellular spine or spur (Spi) and becomes 
thus a terminal armature of the latter. The gland cell may lie within the 
spine (GlCl), ox it may become so greatly enlarged that its base projects 
eneath the inner end of the spine. In the spine tj^e of apparatus the 
terminal seta in some cases is a long, piercing needle, in others it is reduced 
to a thick apical point. In many of the Liparidae and in some of the 
Notodontidae, including the caterpillar of the brown-tail moth Buprociis 



THE BODY WALL AND ITS DERIVATIVES 


59 


chrysorrhea, the seta is replaced by a group of small spicules, which, 
Gilmer believes, are derived from an original branched seta, the main 
shaft of which has been shortened and finally obliterated. A spicule of 
the brown-tail moth caterpillar is composed of a series of dartlike pieces, 
each of which is inserted by its tapering base into the larger distal end 
of the piece proximal to it. 

The poison cell of the stinging apparatus (Fig. 30 A, GlCl) extends 
into the seta along with the trichogen cell, and Gilmer suggests that it is 


/-a 



Fig. 30. — Poisoa setae of certain caterpillars. (Diagrams based on figures from 
Gilmer, 1925.) A, typical structure of a poison seta, with a large gland cell (GlCl) 
discharging through broken tip of seta. B, a poison spine, or multicellular process armed 
with a stinging seta. 

a "sister cell" of the latter, the two being formed probabl}'' by the division 
of a single primary seta-forming cell. The gland cell is always dis- 
tinguished by its greater size, and particularly by its large, irregular, 
branched nucleus. The trichogen commonly disintegrates between 
moults and is often inconspicuous or not discernible in sections. Setal 
glands have been observed on the larvae of other insects than 
Lepidoptera. Woods (1929) says that all the setae of the larvm of the 
alder flea beetle have, in addition to the trichogen, a gland cell that 
opens through a pore on the tip of the seta. 

3. SENSE ORGANS 

The reaction of a complex animal to the environment, that is, its 
adjustment to external changes by movement or other forms of response 
activated from within, is dependent on two accessory conditions. First, 
the outer surface of the animal must be in part at least "sensitive” to 
environmental changes; second, the sensitive areas must be in connection 
with the motor mechanism. The first condition is realized in most 
Invertebrata by the special development of cells of the body wall that arc 
particularlj'^ sensitive to external stimuli (Fig. 31, SCI)’, the second is 



60 


PRINCIPLES OF INSECT MORPHOLOGY 


established by the propagation of nerve tracts that go either directly or 
indirectly from the receptive parts to the motor tissues, or effectors. In 
all but the lowest Metazoa the sensory nerves from the receptor organs 
proceed to a central nerve organ, which in arthropods is the brain or the 
ganglia of the ventral nerve cord {VNC), and here make a connection 
(synapse) with the roots of the motor nerves {MNv) which proceed 
outward to the muscles {Mcl) or other effectors. 

Both the sensory cells and the sensory nerves are derived from the 
ectoderm, as are also the motor cells and the motor nerves ; but, whereas 
the motor nerve tissues lose their connections with the exterior (see page 



Fig. 31. — Diagram of the relation of the epidermal sense cells to the central nervous 
system and the motor mechanism. Epd, epidermis; MCI, motor cell; Mcl, muscle; 
MNv, motor nerve; SCI, sense cell; SNv, sensory nerve; 50, sense organ; VNC, ganglion of 
ventral nerve cord. 

30), the sensory tracts maintain their continuity with it in the sensory 
cells of the epidermis. The body wall in the immediate neighborhood 
of a sensitive cell, or group of such cells, is usually modified to form a more 
efficient receptor apparatus, designed to admit some particular kind of 
impinging stimulus, or a certain group of stimuli; and these special 
receptors constitute the so-called sense organs of the animal (Fig. 31, SO). 
The sense organs of insects are widely distributed over the surface of the 
body and the appendages, and they occur also in the anterior and pos- 
terior parts of the alimentary canal. Each is a more or less complex 
structure elaborated from the several layers of the body wall, the various 

forms of which and their possible functions will be discussed in Chap. 
XVII. 


4. ECTODERMAL GLANDS, CORPORA ALLATA, OENOCYTES 

The ectoderm is the seat of much cellular activity, mostly of a secre- 
tory nature. All the ectoderm cells produce the cuticular substances, 
w ich, being largely nitrogenous compounds, are regarded by some 
VTiters as excretory products. In addition to the general chitinogenous 
function, however, many cells or groups of cells have highly specialized 



THE BODY WALL AND ITS DERIVATIVES 


61 


secretory functions, and in these cells are elaborated a great variety of 
substances which are discharged at the exterior or into invaginations 
of the body wall. The true gland cells of the body wall always remain 
in anatomical continuity with the epidermis, though their bases may push 
far into the body cavity, or the cells themselves may be carried inward 
by deep invaginations of the integument. On the other hand, certain 
cells are given off from the epidermis into the body cavity where they 
become free internal organs. Such cells form the corpora allaia in the 
head, and the oenocytes in the abdomen, which are possibly of the nature 
of endocrine glands (see Chap. XIV). 

The glands of the ectoderm discharging their secretion externally 
are too numerous to be described here in detail. Thej’- arise from all 
parts of the body wall, from the stomodaeal and proctodaeal sections 
of the alimentary canal, and from the ectodermal ducts of the reproduc- 
tive organs. Classified according to their function they include salivary 
glands, silk glands, wax glands, lac glands, food glands, trophallactic 
glands, scent glands, adhesive glands, excretory glands, poison glands, 
stinging glands, defensive glands, repellent glands, moulting glands, 
colleterial or egg-covering glands, mucous glands, and others. 

Structurally the ectodermal glands are specialized cells of the epider- 
mis or of the walls of ectodermal invaginations. Gland cells are usually 
distinguished by their large and often irregular or branched nuclei, the 
nuclei being probably the source of the substances that activate the 
secretory properties of the cytoplasm. The simplest gland form con- 
sists of a single cell, but the majority of insect glands are multicellular. 

Unicellular Glands , — A one-celled gland is usually of greater size than 
the cells surrounding it and, in its simplest form (Fig. 32 A), discharges 
its products directly through the covering cuticula. A larger glandular 
area may include a group of secretory cells (B). Some writers have 
claimed that the cuticular covering of such glands is penetrated bj' fine 
pores, but in most insect glands the secretion escapes by diffusion through 
the very thin cuticula covering the surface of the cells. In manj’’ glands, 
however, a minute cuticular ductule extends from the e.xterior into the 
body of each cell (C, H, a), thus allowing the secretion to pass out through 
an extremely thin layer of cuticula. Unicellular glands of this kind often 
have the distal end of the cell dravTi out into a slender neck, or duct 
(C, Dei). 

Multicellular Glands . — ^The man 5 ’'-cellcd glands maj'' be, as we have 
just noted, merelj'’ a group of cells situated at the surface of the body 
(Fig. 32 B); but most of them are invaginations of the body wall. A 
simple multicellular gland is a mere tube of secretory cells lined vilh a 
delicate cuticular intima (D). Such glands are sometimes evcrsible. 
By a specialization in function between its outer and inner parts a tubular 



52 PRINCIPLES OF INSECT MORPHOLOGY 

gland may become differentiated into a duct (E, Dc{) and a true glandular 
part {01), while a widening of the duct may constitute a reservoir (F, Res) 
for the storage of the secretion products. Glands are frequently 
branched, the branches in some cases being long and tubular and in 
others sacculated at the ends, giving the gland a racemous structure (G). 
In all forms of multicellular glands the intima is continuous over the 
inner surfaces of the cells, but in some it gives off minute capillary 
ductules into the individual cells (H), as in some of the unicellular glands 

(C). 



Pio. 32. — Various structural types of ectodermal glands, diagrammatic. 

6. MUSCLE ATTACHMENTS ON THE BODY WALL 

A most important mechanical feature of arthropod organization is the 
intimate connection between the body wall and the muscles. Yet, in 
their origin, the epidermis and the muscle tissue are entirely distinct, the 
first being derived from the ectoderm, the second from the mesoderm. 
In the embryo the mesodermal cells of the developing muscle fibers attach 
themselves to the inner face of the epidermis (Fig. 33 B), and in some 
larval insects (C, D) this condition appears to be preserved, though the 
basement membrane disappears at the end of the muscle and becomes 
continuous with the sarcolemma of the latter. In postembryonic stages 
of most insects, however, the muscle fibers are attached to the cuticula 
by fine connective fibrils, tonofihrillae (A, Tfhl), that traverse the epider- 
mal layer. The dilator muscles of the ectodermal parts of the alimentary 
canal are said by Boelitz (1933) to be inserted in the same manner by 
tonofibrillae attached on the cuticular intima. 

The tonofibrillae are produced by a transformation of the epidermal 
cells at the ends of the muscles into cuticular fibrils that are continuous, 



63 


THE BODY WALL AND ITS DERIVATIVES 

on the one hand, with the cuticula and, on the other, with the muscle 
fibrillae. The plasmatic parts of the cells maj’’ entirely disappear, but 
in most cases nucleated remnants of the cells are to be seen between 
groups of the tonofibrillae (Fig. 33 F) or at the end of the muscle tissue 
(E, Nu). The striated part of the muscle, according to recent investi- 
gators (Munscheid, 1933; Boelitz, 1933), ends with a Q-disc (A, Q), 
though other writers have claimed a Z-disc is final. At each moult the 
muscles are detached from the tonofibrillae, which are discarded with the 
cuticula. The epidermis at the end of a muscle is renewed either from 



Fig. 33. — Muscle attachments on the body Tvall. A, diacram showinn tonofibrillae 
{Tfbl) traversing the epidermis from muscle to cuticula. B, muscle tissue in cmbrj’o of 
Periplanda attached to epidermis. {From Henneguy, 190G.) C, end of larval muscle of 
Phicgothontius sexta. D, muscles of larva of Balaninus caryac attached on fold of body 
wall. E, attachment of labial muscle of a dragonfly larva, tonofibrillae stained dark. 
{From Munscheid, 1933.) F, muscle attachment in adult Chrysoholhrus femorata. {From 
IF. L. Tower, 1906.) 

persisting cell remnants or from the surrounding epithelium, and, as 
shown by Munscheid, a new set of tonofibrillae is generafed. 

It frequently appears not only that the tonofibrillae traverse the 
epidermal layer, but that they penetrate a varjing distance into the 
cuticula, even to the outer part of the latter (Fig. 33 F). In this case 
it must be supposed, as pointed out by Henneguy (1906), that the 
tonofibrillae, being differentiated at an early stage in the formation of the 
cuticula, are first connected vith the outer layers of the exocuticula, and 
their proximal extensions are then imbedded in the endocuticula sub- 
sequently laid down beneath the former. 



64 


PRINCIPLES OF INSECT MORPHOLOGY 


6. MOULTING 

The nonelastic nature of the arthropod cuticula gives the body wall 
but little tensibility. When the cuticula is once formed, therefore, the 
integument can ordinarily increase in extent only in so far as the wrinkles 
and folds of the cuticula may be straightened out. Before reaching 
mature size all arthropods cast off the cuticula at intervals and thus 
release the epidermis from the limitations of its external covering, allow- 
ing the epidermal cells to undergo a brief period of development while a 
new cuticula is being formed. 

The shedding of the cuticula is known as moulting, or ecdysis. Moult- 
ing affects the entire body wall and all internal parts that are formed as 
invaginations of it. The discarded slough constitutes the exuviae. 
(This word in its Latin usage has no singular form ; “ exuvium,” sometimes 
used, is without grammatical standing.) 

The succession of ecdyses divides the life span of the animal 
into a series of stages, while the animal itself appears as a series of instars. 
The number of moults varies with different species or groups of insects 
and is frequently different with individuals of the same species reared 
under the same conditions. It is influenced somewhat by temperature, 
humidity, and the amount of feeding. Yet, notwithstanding all irregu- 
larities, the number of moults is surprisingly constant for each species 
and may be characteristic of families and even orders. Most insects 
moult from four to six times before they become mature ; some normally 
shed the skin only two or three times or but once, but only abnormal 
conditions induce a very large series of moultings. The majority of 
pterygote insects do not moult after reaching the adult form; most of 
the mayflies, however, undergo a complete ecdysis shortly after becom- 
ing winged, and some of the Apterygota moult irregularly throughout 
life, as do many arthropods other than insects. 

The begmning of an instar is not marked by the discarding of the 
old cuticula, though in “life-history” studies the length of a develop- 
mental stage is usually measured from the time the exuviae are cast. 
Physiologically, however, it should be reckoned from the time the old 
cuticula is loosened from the epidennis, which more approximately 
marks the beginning of the short period of development that is to give 
the increased size and the characteristics of the following instar. The 
loosened cuticula may not be shed for several days, and in some cases it 
remains intact as a protective capsule about the insect through a part 
or all of its subsequent development. When the cuticula begins to 
separate from the epidermis preparatory to ecdysis, the insect usually 
ceases to feed and becomes more or less quiescent. Each active stage in 
the insect’s life is thus followed by a sluggish premoulting period. 



THE BODY WALL AND ITS DERIVATIVES 


Go 


Ecdysis begins with a splitting of the old cuticula, witliin which 
is contained the new instar of the insect. The cleft usually forms over 
the forward part of the body and over the top of the head, but the 
details vary much in different insects. The released insect issues from 
its covering as quickly as possible before the latter becomes dry. At 
least a part of the cuticular linings of the tracheal tubes and the sto- 
modaeal and proctodaeal parts of the alinientarj^ canal are usually 
drawn out attached to the slough from the body wall, but it is not clear 
just how the intima of the minute ramifications of the tracheal sj’stcm 
is cast out. The cuticular intima of the stomodaeum of the cockroach, 
Eidmann (1924) has shown, is broken in the region of the foramen 
magnum, and only the head part is drawn out with the cuticula of the 
body wall; the rest, as is also the intima of the proctodaeum, is frag- 
mented and discharged through the alimentar 3 ’’ canal. 

The cuticula at ecdysis evidentlj’’ ruptures along predetermined 
lines of weakness, though the splitting may be expedited bj- muscular 
contractions that produce contortionistic movements of the l^odj'. 
When the rupture in the old cuticula is once formed, the insect, in 
many cases, appears automatically to swell out from the cleft. The 
enlargement of the body is produced bj’’ the taking of air or water into 
the alimentary canal through the mouth. Generallj' it is the eroj) that 
is distended. The air or liquid must enter the alimcntaiy canal cither 
around the stomodaeal lining being discarded or through a rupture in 
the latter. Eidmann (1924) points out that the breaking of the old 
stomodaeal intima in the back part of the head in the cockroach allows 
air to be swallowed into the lumen of the crop. He fulh' demonstrates, 
moreover, that the presence of air in the crop is a neccssaiy condition 
for successful ecdysis in the cockroach. 

The mechanism of moulting is perhaps not jmt entirelj' undcr.sfood, 
but the weight of evidence seems to uphold the current view that the 
separation of the old cuticula from the epidermis is accomplished ly a 
moulting liquid formed bj'- the epidermal cells, or 1y special cxnvinl 
glands of the epidermis, that dissolves the inner laj’crs of the endocuticula 
and thus frees the rest of the cuticula from the cellular matrix. It is 
true at least that in manj’’ insects a copious liquid appears beneath the 
loosened cuticula just before ecdj'sis, and that special glands apj)e.ar 
in the epidermis and become actiA'o at the time of the moult. 

Glands of the epidermis supposed to secrete the exuvial liquid have 
been described in Apterj'gota, Hcniiptcra, Neuroplcra, Coleoptera, 
Tepidoptera, and H 3 Tnenoptcr 3 , in .some cases arranged scgmentalh*, in 
others scattered over the general surface of the bod 3 '. Ihc 3 ' are func- 
tional during larval stages and ma 3 * be carried over into the pupa, but 
the 3 ' are absent in adults of pter 3 'gote insects. In Collcmbola, which 



66 


PRINCIPLES OF INSECT MORPHOLOGY 


moult during the adult stage, the glands are said to persist throughout 
life. Some writers, however, particularly von Buddenbrock (1930, 1931) 
and Hoop (1933), claim that these supposed exuvial glands do not produce 
the moulting fluid, which, they believe, is secreted by the cells of the 
general epidermal epithelium. The relative scarcity of the glands in 
most cases and their absence in certain parts of the body, as in the head, 
the appendages, and the tracheae, would seem to indicate that they are 
not adequate to furnish the large amount of subcuticular fluid that 



Fig 34. — Examples of moulting glands. A, one-celled epidermal gland of a collem- 
bolan, Neanura muscorum. (From Philiptachenko, 1907.) B, C, D, Versonian glands of 
caterpillars. (From Plotnikow, 1904.) B, abdominal gland of Bomhyx mori at second 
moult; C, abdominal gland of Ocneria monacha; D, third thoracic gland of Ocneria dispar 
at fifth moult. E, larva of Leptinoiarsa about to pupate, with epidermal gland cells 
beneath newly forming cuticula. (From W. L. Tower, 1906.) F, moulting gland of larva 
of Altica bimarginata at second moult. (From Woods, 1929.) G, H, a moulting gland at 
two stages of ecdysis in Rhodnius xrroUxua, (From Wigglesworih, 19336.) 

appears at ecdysis. Hoop concludes that moulting results from a 
rhythmically repeated activity of the epidermal cells, causing them to 
secrete first the exuvial fluid and then the new cuticula. 

Epidermal glands to which the formation of a moulting liquid is 
ascribed were first described in the silkworm by Verson (1890), and these 
glands of lepidopterous larvae are known as the Versonian glands. 
They are three-celled structures of various shapes (Fig. 34 B, C, D) 
situated along the sides of the body, two on each side of each thoracic 
segment, and one on each of the first nine abdominal segments. The 
function of the Versonian glands in connection with moulting has been 
questioned by von Buddenbrock (1930), who claims that the outlet 
ducts open at the surface of the old cuticula, and that the secretion, 
therefore, could not have a dissolving effect on the inner layers of the 
latter. Furthermore, since he finds that the glands do not reach their 



THE BODY WALL AND ITS DERIVATIVES G7 

maximum secretory activity until ecdj^sis is completed, he postulates 
that the large inner ceil is an organ of internal secretion that activates 
the moulting process. The moulting fluid of lepidopterous larvae, 
von Buddenbrock believes, is formed by the ordinary’ epidermal cells 
and not by the glands. Wachter (1930), however, figures the \*crsonian 
glands of the silkworm as opening beneath the cuticula, and it is perhaps 
possible that the vacuolization of the active inner cells of these organs 
observed by von Buddenbrock is a final degenerative process. 

The supposed moulting glands of some insects arc said to consist 
each of a single specialized epidermal cell (Fig. 34 A, E). According to 
Tower (1906), the exuvial glands of the potato beetle (Lcplinolarsa) 
are of this kind (E), being formed during embryonic and larval stages 
by a modification of certain cells of the epidermis; after each ccdj’.'^is 
the gland cells rapidly degenerate and finally disappear. Woods (1929) 
describes the moulting glands of the alder flea beetle (AUica) as com- 
prising each three ceils (Fig. 34 F), two of which, however, arc .^mall 
and serve merely as neck or guard cells, while the large third cell (GlCl) is 
the true glandular organ. The gland cell contains a branched canaliculus 
(Dei) through which the exuvial fluid is discharged beneath the old 
cuticula as the new is being formed. These glands are found to persist 
from the embryo to the pupal stage, becoming functional at each cedysis. 
Wigglesworth (19336) gives a concise description of the development 
and secretory activity of one-celled epidermal glands in the hemipteron 
Rhodnnis prolizus, which, he says, are without doubt responsible for the 
formation of the moulting fluid. The glands (Fig. 34 G, H, GlCl) have 
distinct ducts that terminate at the surface of the epidermis beneath 
the cuticula. They are formed anew at each moult from undifTerentiated 
cells in the epidermis and are functional only during moulting; after 
eedysis they break down, their nuclei undergoing chromatolysis. New 
glands do not appear in the adult stage. 

Little is known concerning the chemical nature of the moulting fluid. 
Verson says the moulting fluid of the silkworm contains a solution of 
oxalic acid salts at the fourth larval moult, and uric acid at the time 
of the moult to the pupa. It is known, however, that products of the 
Malpighian tubules discharged from the anus may find their way beneath 
the loosened cuticula. Wigglesworth (19336) finds that the moulting 
fluid of Rhodnius is a neutral liquid, which appears to be free from chloride 
end gives no precipitate with silver nitrate and nitric acid but show.s 
protein color reactions. 

There is evidence that moulting is induced by a moulting hormone 
produced within the head, possibly, as suggested by V igglcsworth (1934), 
secreted by the corpora allata. It has been found that tran.'fu.'-ion of 
Wood from an insect about to moult into anotiicr not yet in a moulting 



68 


PRINCIPLES OF INSECT MORPHOLOGY 


condition will induce ecdysis in the latter. Wigglesworth has demon- 
strated that in the case of the hemipteron Bhodnius prolixus an insect 
with its head cut off prior to a certain period before the moulting time, 
though it may live indefinitely, will not normally moult, but it can be 
caused to moult by transfusion of blood into its body from an insect 
after the “critical” period. 

Since the somatic muscles must be detached from the cuticula of the 
body wall at each moult, it is necessary that they become reattached 
to the newly forming cuticula. The new attachments, Munscheid (1933) 
has shown, in dragonfly larvae are formed by new tonofibrillae differen- 
tiated in the regenerated epidermis at the end of the muscle, not as 
Tower (1906) supposed by the imbedding of the old tonofibrillae in the 
new cuticula. If the formation of new muscles takes place at the time 
of moulting, these muscles may become attached to the cuticula in the 
usual manner. But in holometabolous insects the imaginal muscles 
are formed after the last larval, or pupal, ecdysis, and Poyarkoff (1914) 
sees in this condition the reason for a subsequent moult, and hence the 
establishment of a pupal stage in the life cycle of insects in which the 
muscles undergo histolysis and regeneration after the end of the larval 
stage. It is clear that another, preimaginal moult then becomes neces- 
sary in order that the new muscles may become affixed to the cuticula. 
Thus Poyarkoff holds that the pupa is a secondary preimaginal stage 
interpolated into the life cycle of holometabolous insects and is not to be 
regarded as representing the last larval or nymphal stage of other insects. 

GLOSSARY OF TERMS APPLIED TO THE BODY WALL 

Alveolus {Alv ). — A hair socket. 

Apodeme (Ap ). — Any cuticular ingrowth of the body wall, usually formed in a 
multicellular matrix, but sometimes in a single cell. 

Apophysis. — Any tubercular or elongate process of the body wall, external or 
internal. 

Articulation. — A movable point of contact between two sclerotic parts of the body 
wall. 

Basement Membrane {BMh ). — ^The inner noncellular membranous lining of any 
epithelial layer. 

Body Wall. (BW ). — The integument of the body, formed of the ectoderm, con- 
sisting of epidermis, cuticula, and basement membrane. 

Chitin. ^The chemical substance that forms the groundwork of the cuticula, but 
not necessarily the principal part of it. 

Cuticula (Ct ). — The outer noncellular layers of the body wall. 

Ecdysis. ^The shedding of the cuticula. {Moulting.) 

Ectodem (F?cd).— The outer embryonic layer from which the epidermis of the 
body wall is derived. 

Endocuticula {Enct ). — The inner softer layer of the cuticula. 

Endoskeleton.- Collectively the internal cuticular, or apodemal, ridges and 
processes of the body wall. 



THE BODY WALL AND ITS DERIVATIVES C9 

Epicuticula (Epct). — ^The nonchitinous external filinlike covcrinp of tlie oxoruticula. 
(GrcndamcUc.) 

Epidennis (Epd). — The epithelium of (he body wall. (Hupodcrmis.) 

Exocuticula (Excl). — The outer chitinous layer of the cuticula, containinp the 
sclerotic deposits of the cuticula when the latter arc present. 

Exoskeleton. — Collectively the e.xternal plates of the body wall. 

Exuviae. — The cuticular parts discarded at a moult. 

Eruvial Glands. — Glands of the epidermis supposed to secrete the exuvial or 
moulting liquid. 

Hypodennis. — See epidermis. 

Moulting. — The periodie process of loosening and discarding the ciiticuia. accom- 
panied by the formation of a new cuticula, and often by structural chango.s in the 
body wall and other organs. 

Moulting Glands. — See exuvial glaiids. 

Scale (<Sg). — A flat unicellular c.xternal process of the body wall, probably a modi- 
fied seta. 

Sclerite. — Any of the large or small sclcrotizcd areas of the body wall. 

Sclerotization. — The hardening of the body wail by" the dcjjosit of sclcrotiring 
substances in the e.xocuticula. 

Sense Organ (SO). — Any specialized innervated structure of the body wall 
receptive to external stimuli; most insect sense organs are innervated setae. 

Seta (Scl). — A hairlike unicellular c.xternal procc.-^s of the body wall or of any 
derivative of the latter. 

Setal Membrane (smb). — The membranous floor of the hair socket, or alveolus, 
supporting the seta. 

Spine. — A multicellular external process of the body wall. 

Spur. — A movable spino-liko process. 

Suture. — Any of the external grooves of the body wall fonning internal eiiticulnr 
inflections, or any narrow membranous areas between sclcritc-'. 

Tonofibrillae (T/hf). — Cuticular fibrils connecting the mu.“clc fibers with the 
inner surface of the cuticula. 

Tormogen {Tmg). — The epidermal cell as.sociated with a seta that forms tlie .‘•ctal 
membrane or socket. 

Trichogen (,Trg). — An epidermal cell that generate,-; a seta. 

Trichopore.— The opening in the cuticula beneath a seta, giving pas.-age to the 
hair-forming process of the trichogen. 



CHAPTER IV 

BODY REGIONS, SCLERITES, AND SEGMENTATION 

An arthropod, in a final anatomical analysis, consists of a cylindrical 
trunk containing the visceral organs and of a series of ventrolateral limbs. 
The part of the trunk traversed by the alimentary canal becomes divided, 
by the process of metamerism, into a succession of somites, or body 
segments corresponding to the series of limbs. Anterior to the first 
somite is the preoral prostomium, and the segmented body terminates 
with an endpiece, the periproct, or telson, which contains the anus (Fig. 
24 B) . The trunk segments, including the prostomium and the periproct, 



Fig. 35. — Diagrams showing the lengthwise regional divisions of an arthropod as deter- 
mined by the position of the limb bases. a-a, dorso-pleural line; b-b, pleuro-ventral line; 
Cer, cercus; Cxpd, limb basis, or coxopodite; D, dorsum; Id, laterodorsum; Ovp, ovipositor; 
P, podial, or pleural, region; jSp, spiracle; Stn, sternum; T, tergum; V, venter. 

become segregated in the adult animal into two or three distinct body 
sections, or tagmata, the limits of which vary in the different arthropod 
groups. Those of the Hexapoda are the head, the thorax, and the abdo- 
men. The limbs are characteristically segmented. Primarily they were 
aU organs of locomotion, but in modern arthropods they are variously 
modified for many purposes. 

The Surface Regions of the Body. — ^The implantation of the bases 
of the limbs along the lower lateral parts of the trunk divides the body 
surface longitudinally into four principal regions, namely, a dorsal region 
lying above the bases of the limbs, a ventral region lying between the 
limb bases of opposite sides, and a ventrolateral region on each side 
containing the limb bases. These regions are respectively the dorsum 
(Fig. 35 A, D), the venter (V), and the podial, or pleural, regions (P). 

70 



BODY REGIONS, SCLERITES, AND SEGMENTATION 


71 


The lateral line on each side between the limb bases and the dorsum is 
the dorso-pleural line (A, B, a~a); that between tlic limb bases and the 
venter is the plettro-venfral line (b-b). There is reason to believe that the 
spiracles, or segmental apertures of the tracheal system (Sp), arc situated 
in the lower lateral parts of the dorsum just above the limb bases. 

Each segment of the trunk presents the same surface regions as does 
the body as a whole. In stud3nng any individual segment, therefore, we 
must distinguish a segmental dorsurn, a segmental venter, and segnnntal 
pleural areas. 

Sclerites. — ^The arthropod integument may remain soft and n('xible 
in all its parts, as in the larvae of man3' insects, but usualh- it is h.ardened, 
or sclerotized, in definite areas owing to the deposit or format if»n of other 
substances than chitin in its cuticula. These hardened areas are the, 
sclerites. 

Sclerites primaril3»’ ma3' be intrasegment al and intersf'gmental and 
ma3’’ occupy any of the several regions of an individual segment. A m.ajor 
segmental plate of the dorsum is a tergum, or notum; a major segmental 
plate of the venter is a sternu 7 n; plates of the pleural areas are designated 
pleural sclerites. Subdivisions of a principal .segmental plate or the 
component sclerites of a major area of sclorotization, then, become 
iergites, sternites, and plcuritcs, respect iveh', since the suffix -ife has a 
fractional significance. Unfortunate^' there is little uniformit3' in the 
usage of these' terms. Some entomologists use the words ‘‘tergum” and 
“sternum” to designate the dorsal and ventral regions of a segment and 
distinguish the regional plates as “tergites” and ‘‘sternite.-;.” Thi.s 
practice, however, leaves us without suitable terms for the minor divisions 
or component sclerites of major areas of sclcrotization. lienee, while the. 
nomenclature given above, and adopted in the pre.«enl text, mav' be 
somewhat arbitrary, it is practicable in application and for the mo.'^t part, 
can be consistentl3^ followed. 

Sclerites do not define anatomical areas. The student nmst recognize 
this fact as fundament.alh^ important. Much inaccuracw of tlnnighi, as 
U’cll as confusion in descriptive statements, has resulted from identif3’ing 
areas of sclorotization with the morphological regions tiu.y oecupN'. 

The pleural regions of the arthropod bod3’ wall are t3'pieall3' mem- 
branous, allowing a free movement to the appendages ari.-ing iroin them, 
as is well illustrated in the centipedes. Pleural sclerites ma3' be devehtjM-d 
in the jileural walls about the bases of the limijs; but in mo.^i cases it 
appears that the major pleural sclorotization in eaeli scgimmt is derived 
from the limb basis, representing cither the entire basis of ibe a}ij>‘'ndag(: 
or the proximal, .subco.xal part of the basis. 

The spiracles are contained in the laieral j^arts of the terga if the 
tergal sclerotizations c.vtcnd downward on the sides to tin' lower litnils oi 



72 


PRINCIPLES OF INSECT MORPHOLOGY 


the dorsum; otherwise they generally lie free in the membranous parts 
of the dorsum below the edges of the terga, though in some cases they are 
contained in lateral tergal sclerites, which may be distinguished from the 
principal dorsal tergites as paratergites, or later otergites. 

Identification of the Body Regions. — In the study of insects it is of 
prime importance to be able to determine the limits of the surface regions 
of the body, and to identify the corresponding parts in the several sections 
of the trunk. The diagram given at C of Fig. 35 shows a typical form 
of the body structure in an immature generalized pterygote insect. The 
spiracles lie between the subcoxal plates of the leg bases on the thorax 
and in the membranous lower parts of the dorsum on the abdomen. The 
series of lateroventral appendages begins on the head with the mandibles 
(or theoretically with the second antennal appendages), followed by the 
first and second maxillae, which are all articulated to the lower edge of 
the cranium. On the thorax are the legs, the bases of which (Cxpd) form 
the subcoxal pleural plates, which in the second and third segments 
are extended upward to the bases of the wings in the adult stage. 
Appendages are generally absent on the pregenital part of the abdomen 
in adult pterygote insects, but the eighth and ninth segments, in the 
female, may bear each a pair of gonopods, which contribute to the forma- 
tion of the ovipositor (Ovp), and the eleventh segment supports the cerci 
(Cer), which are known to be true appendages. 

If the dorso-pleural and pleuro-ventral lines are not marked by 
evident structural features, their positions can be determined by applying 
the principles already explained. The dorso-pleural line (Fig. 35 C, 
a-a) begins on the head between the bases of the gnathal appendages 
and the lower edge of the cranium; on the thorax it goes over the subcoxal 
plates of the leg bases but dips down between them to pass beneath the 
spiracles, since the latter belong to the dorsum; on the abdomen it 
follows a straight course through the lateral membrane below the spiracles 
and finally runs out dorsal to the base of the cercus. The pleuro-ventral 
line (6-6) begins on the head mesad of the bases of the gnathal append- 
ages; on the thorax it separates the leg bases from the primary sternal 
areas; on the abdomen it traverses the lateral parts of the definitive 
ventral plates until it comes to the genital segments, where it passes 
beneath the bases of the gonopods, and then finally ends between the 
base of the cercus and the paraproct. 

The corresponding surface regions of the head, the thorax and the 
abdomen can now be identified with one another. The dorsum is the 
entire surface above the dorso-pleural lines, containing the spiracles 
and the segmental terga of the thorax and abdomen and the principal 
part of the cranial capsule of the head. The venter is the region below 
the pleuro-ventral lines, including the under surface of the head between 



BODY REGIONS, SCLERITES, AND SEGMENTATION 73 

the gnathal appendages, and the true sternal parts of the thorax and 
abdomen. The pleural region is that containing the bases of the gnathal 
appendages, the subcoxal plates of the leg bases, the lateral parts of the 
pregenital ventral plates of the abdomen, the bases of the gonopods, 
and the base of the cercus. The ventral plates, or definitive sterna, 
of the pregenital part of the abdomen are thus seen to be really pleuro- 
sternal plates, since they cover the areas of the primitive limb bases 
{Cxpd) and those of the primary sterna (Stn). In some of the Apterygota 
the limb bases of the entire abdomen are plates distinct from the primitive 
sterna and bear appendicular parts of the limbs in the form of styli; in 
others the styli are borne by lateral parts of the definitive sterna, which 
are thus seen to be composite 
pleurosternal (that is, coxoster- 
nal) plates. 

Segmentation. — In soft-bodied 
segmented animals, as in the 
annelid worms and in the worm- 
like larvae of certain insects, the 
segmental regions of the body 
(Fig. 36 A, Seg') are separated by 
circular constrictions of the integ- 
ument (Isg). Internally the 
Intersegmental grooves form folds, 
and on these folds are attached 
the fibers of the principal longi- 
tudinal bands of somatic muscles 
{LMcT). Animals having this 
t3T3e of structure can bend the 
body freely in any direction, and 

they can shorten it by a length- Fig. 36. — Types of body segmentation. 

. ... „ , A, primary segmentation. B, C, secondary 

WISG COHtrRCtiOIl 01 tllG SG^mGIluS. sGgniGiitation. A.c, antGCOSta; dcs, antG- 
In the typical adult arthropod costal suture; ast, acrostermte; atg, acroter- 
, , , p gite; Isg, intersegmental fold; LMcl, longi- 

structure the segmental areas OI tudinal muscles; Mb, conjunctival, secondary 

the body are hardened by the de- intersegmental, membrane; Rd, posterior fold 

. . . „ , ... , . of secondary segment; Seg , primary segment 

position of sclerotizing substances (somite); Seg", secondary segment. 

in the cuticula, forming usually 

tergal and sternal plates (B, T, Stn). The areas of sclerotization, however, 
do not coincide with the areas of the original segments, since they do not 
cover the posterior parts of the latter and may extend anteriorly a short 
distance before the intersegmental grooves on which the muscles are 
attached. The trunk thus becomes differentiated into a series of sclerotic 
annuli, the scleromata (B, Seg"), and intervening membranous conjunc- 
iivae (Mb). The former not only are movable upon each other by reason 




74 


PRINCIPLES OF INSECT MORPHOLOGY 


of the flexible conjunctivae but can be partially retracted each into the 
posterior end of the one preceding by the contraction of the longi- 
tudinal muscles attached upon them (C). 

On comparing the two kinds of organization described in the preceding 
paragraph, it becomes evident that we must distinguish two types of 
segmental structure included under the general term “segmentation.” 
The first type (Fig. 36 A), which occurs in all soft-bodied arthropods and 
in annelid worms, is the embryonic form of segmentation. It is, 
therefore, the more primitive one, and we may designate it 'primary 
segmentation. The other type (B, C) is clearly a secondary differentiation 
of the body into successive parts by the formation of plates in the integu- 
ment alternating with nonsclerotized areas. This type of body segmen- 
tation we may distinguish as secondary segmentation. 

Primary Segmentation. — In soft-bodied larval insects, as in the annelid 
worms, the principal longitudinal muscles are attached typically on the 
primary intersegmental folds. It is evident, therefore, that there is a 
close relation between this more primitive form of body segmentation 
and the segmentation of the muscle-forming parts of the mesoderm into 
myotomes. We may, then, define as primary segmentation that form of 
segmentation (Fig. 36 A) in which the functional intersegmental lines 
of the body wall (Isg) coincide with the lines of attachment of the princi- 
pal longitudinal muscle fibers (LMcl). The segments in this type of 
segmentation (Seg') correspond with the true somites, or embryonic 
metameres. 

Secondary Segmentation.- — ^The development of hardened areas or 
plates in the body wall, a feature distinguishing most of the arthropods 
from the annelids, was perhaps in the first place a protective device. 
But, since in the arthropods the muscles have their attachments on the 
body wall, an advantage is gained if the muscles are affixed to the integu- 
mental plates, because the latter become, in this case, not only protective 
coverings but elements of the motor mechanism as well. Hence the 
sclerotized areas of the arthropod body wall, such as the major tergal 
and sternal plates (Fig. 36 B, T, Stn), usually include the parts of the 
primary intersegmental folds (A, Isg) on which the longitudinal muscles 
are attached. In order to retain the power of motion, however, there 
must be left a flexible nonsclerotized area (B, Mh) at the opposite end of 
the segment. These flexible conjunctival areas of the integument now 
become the functional intersegmental membranes. For this reason the 
lunits of the definitive segments in arthropods that have body-wall plates 
(B, Seg ) are not coincident with the primary intersegmental folds 
(A, Isg). The areas of flexibility between the sclerotic parts of the 
segments (B, Mh) divide the body in a new way, which is clearly secondary 
and therefore constitutes a secondary segmentation. In primary seg- 



BODY REGIONS, SCLERITES, AND SEGMENTATION 


75 


mentation the longitudinal muscles are intrasegmental, inasmuch as they 
are coextensive with the segmental areas of the body (A.)^ in secondary 
segmentation the muscles become intersegmental (B). The flexible 
areas of the body wall in secondary segmentation (B, Mh) are usually the 
posterior parts of the primary segments. This arrangement allows the 



Fig. 37. — The body sections (tagmata) of an insect and their typical segmentation. 
Note free intersternites (list, 2Ist) in the thorax, and reversed overlapping of the thoracic 
sterna. 

muscles of each segment to draw the following segment forward (C), and 
the infolding of the conjunctival membranes gives the characteristic 
posterior overlapping to the successive segmental plates. 

In adult insects, secondary segmentation in its typical form prevails 
throughout the length of the abdomen, where the limits of the tergal 
and sternal plates coincide morphologically with each other (Fig. 37, A6). 



Fig. 38. — Ventral surfaces of two body segments of a chilopod, showing alternating 
segmental sternal plates {Stn) and intersegmental intersternites ilst). 


In the thorax (Th), however, the terga, though fundamentally secondary 
segmental plates of the usual form, may undergo modifications that alter 
this structure, and the thoracic sternal sclerotizations are characteristic- 
ally of a different type of structure, which is best illustrated in the 
Chilopoda. 

The secondary segmentation of the chilopods has produced typical 
secondary segmental plates only in the dorsum. In most of the centi- 


76 


PRINCIPLES OF INSECT MORPHOLOGY 


pedes, excepting Scutigevcij the ventral sclerotizations throughout the 
length of the body have taken the form of independent segmental and 
intersegmental sclerites, the former occupying the primary segmental 
regions, the latter being confined to the intersegmental folds (Fig. 38, 
Stn, 1st). In some families the ventral longitudinal muscles retain their 
attachments on the intersegmental plates, in others they have migrated 
to the segmental plates, or to supporting arms or ligaments of the latter. 
This same type of sclerotization occurs in the venter of the thorax of 
many insects, in which there is a small intersternite (Fig. 37, list, 2Ist) 
situated posterior to the principal sternal plate of the prothorax and the 
mesothorax {Stni, Stn 2 ). 

Structure of a Typical Secondary Segment. — A typical segment of an 
adult arthropod is in general one of the secondary annular sections of 
the body defined by the lengthwise extent of its dorsal and ventral plates 
(Fig. 36 C). A considerable part of the true segmental area, however, 
is formed by the posterior conjunctival membrane (B, C, Mb), which is 
usually infolded and more or less concealed within the posterior ends 
of the tergal and sternal plates (C). The wall of each segment, as we 
have seen, contains typically a dorsal sclerotic area, or tergum (B, T), 
and a ventral sclerotic area, or sternum (Sin), while the lateral or pleural 
walls may contain each one or more pleural sclerites. 

The Tergum. — In its typical form the back plate of a secondary seg- 
ment includes the sclerotization of the dorsum of the primary interseg- 
mental area preceding (Fig. 36 B, T). The primitive intersegmental fold 
(A, Isg), therefore, becomes a submarginal ridge near the anterior edge 
of the inner surface of the tergum (B, Ac). This ridge is the antecosta 
of the tergum. The corresponding external groove, or transverse line of 
inflection forming the antecosta, is the antecostal suture (acs). The 
narrow precostal lip of the tergum is the acrotergite {atg) . The postcostal 
tergal sclerotization usually forms a simple plate, which is subject to 
modifications in various ways, especially in the wing-bearing segments of 
pterygote insects, but it may be broken up into smaller sclerites, as in 
many holometabolous larvae. The dorsal longitudinal muscles, pri- 
marily attached on the intersegmental folds (A), usually retain their 
attachments on the antecostae in the secondary segments (B, C), though 
some or all of their fibers may migrate to the precostal or postcostal areas 
of the tergum. 

Tergal plates are present in the adult stages of nearly all arthropods. 
Their principal variation in size with relation to the shape of the segment 
is in a transverse direction. They may be limited to the median part 
.-■>1 the back, or their lateral areas may be extended downward a varying 
distance in the lateral walls of the dorsum, and the lateral margins may 
project as free folds either horizontally extended or ventrally deflected 



BODY REGIONS, SCLERITES, AND SEGMENTATION 


77 


over the sides of the segment, sometimes concealing the bases of the legs. 
Frequently lateiodorsal sclerites are distinct from a principal median 
tergite. 

In the thoracic segments of winged insects the typical structure of the 
dorsal plates is generally obscured by a modification in the intersegmental 
sclerotization, correlated with the development of the wings as efficient 
organs of flight. The intertergal parts of the conjunctivae between the 
mesothorax and metathorax and between the metathorax and first 
abdominal segment are almost obliterated by an anterior extension of 
the acrotergites of the metathoracic and first abdominal terga (Fig. 39 B). 
Furthermore a secondary membranous suture may appear in the tergal 




Fig. 39. — Diagrams showing intersegmental relations of the skeletal plates in 
secondary segmentation. A, generalized condition. B, specialized condition in the 
dorsum of the thorax of winged insects, in which the enlarged acrotergites (A, atg) become 
the postnotal plates (B, PN) of the second and third segments. 

region just behind the antecosta (Ac) of each of these segments. There 
is thus formed posterior to each wing-bearing tergal plate {T 2 , T 3 ) a, 
postnotal plate (PN 2 , PN 3 ) bearing the intersegmental attachments of the 
dorsal muscles (DMcl). Since the dorsal muscles are greatly enlarged 
in the thorax, the antecostae develop large lobes, the phragmata (Ph), 
for their accommodation. The postnotal plates are usually regarded as 
belonging to the segment preceding in each case, but it is clear that they 
are intersegmental structures analogous to the intersternites of the venter 
between the prothorax and mesothorax and between the mesothorax and 
metathorax (A, 1st), which are usually more closely associated with the 
preceding sterna (Fig. 37, list, 2Ist). 

The Sternum . — Sternal plates are not so constant a feature of the 
arthropod skeleton as are the tergal plates. They may be present or 



78 


PRINCIPLES OF INSECT MORPHOLOGY 


absent within the same major group, and, where present, they are often 
highly variable in both shape and size between closely related forms, 
and even in different body regions of the same species. 

The ventral plate of a segment usually has the same structure as 
the tergum (Fig. 36 B, Stn), as in the abdomen of insects (Fig. 37, Ab), 
where the sternum generally is an inverted replica of the tergum. In this 
case each sternum bears anteriorly a submarginal antecosta on its inner 
surface (Fig. 36 B, Ac), on which the ventral longitudinal muscles are 
usually attached, and presents a narrow precostal lip, or acrosternite (ast), 
corresponding to the acrotergite of the dorsal plate. In the thorax of 
many insects, however, the intersegmental sclerotizations of the venter 
form small independent plates, or intersternites (Figs. 37, 39 A, 1st), 
as they do in the whole length of the body in most of the chilopods (Fig. 
38). The intersternites of the thorax of insects, which may occur between 
the prothorax and mesothorax and between the latter and the metathorax, 
are known as spinasierna because each usually bears an internal spine-like 
process giving attachment to some of the ventral muscles. The spinas- 
terna are often united with the segmental sterna preceding them, but 
never with those following. A definitive thoracic sternum, therefore, 
never has a true antecosta. Most of the ventral muscles of the thorax 
are attached (probably secondarily) on apodemal processes of the seg- 
mental plates; a few fibers, however, usually retain the primitive con- 
nections with the median processes, or spinae, of the intersegmental 
spinasterna (Figs. 87, 103 A). 

The definitive sterna of insects are usually composite plates, each 
comprising a median region representing the area of the primitive seg- 
mental sternum and lateral parts derived from the limb bases. In the 
thorax the adjoined lateral elements are the ventral arcs of the subcoxal 
parts of the leg bases ; in the abdomen the sternal plates appear to contain, 
in most cases, the entire basal parts of the otherwise suppressed limbs. 
The term “sternum” is usually applied to the principal ventral plate of a 
segment regardless of the real or theoretical composition of the latter. 

The Pleural Sclerites . — ^The podial areas of arthropod body segments, 
that is, the so-called pleural areas in which the limbs are implanted, are 
usually membranous and seldom contain any extensive sclerotization 
that can be attributed to the wall of the body itself. The proximal, 
or subcoxal, parts of the limb bases, however, are often expanded in the 
pleural wall and separated by an articular ring from the coxal parts of 
the limbs. The coxae thus become the functional bases of the append- 
ages, and the subcoxae serve as supports for the latter in the podial areas 
of the body wall. Wherever sclerotizations occur in the podial areas, 
therefore, they appear generally to be derived from the bases of the 
limbs, though it is probable that certain small sclerites may belong also 



BODY REGIONS, SCLERITES, AND SEGMENTATION 


79 


to the region of the peripodial membranes. All sclerotizations of the 
podial areas of the body segments, however, are in general termed 
pleurites. In the thorax of pterygote insects the subcoxal sclerotization 
above, before, and behind the coxa is knovm as the pleuron. 

Subcoxal pleurites occur in most of the body segments of the Chilo- 
poda (Fig. 52 A, Sex), where they have the form of small sclerites of 
various shapes more or less closely associated vdth the bases of the coxae. 
Similar sclerites are present in the pleural areas of the thorax in apterygote 
Hexapoda, but here they frequently appear as two crescentic arches over 
the bases of the coxae (Fig. 89). In adult pterygote insects the thoracic 
pleural plates are much enlarged, especially in the wing-bearing segments, 
where they form supports for the wings as well as giving articulation to 
the coxae of the legs. In the decapod Crustacea the inner walls of the 
gill chambers are evidently of subcoxal origin, but in most of the Crus- 
tacea and in the Arachnida there is little evidence of the presence of 
pleurites derived from the limb bases. 

In the abdominal segments of insects the limb bases are sometimes 
represented by distinct plates occupying the pleural areas between the 
terga and sterna, as in certain Thysanura (Fig. 138 A, Cxpd) and in 
many larval forms (Fig. 150 A, Cxpd)] in the genital segments they are 
retained as the basal plates of the gonopods (Fig. 35 C). In general, 
however, the abdominal pleurites appear to be fused vdth the primitive 
sterna in continuous plates, which are the definitive sterna. 

Intersegmental Relations. — The primary intersegmental grooves, we 
have seen, are the functional segmental limits only in soft-bodied arthro- 
pods or in forms mth but a weak or partial sclerotization of the integu- 
ment (Fig. 36 A). In all arthropods with well-developed body-wall 
plates, the definitive segmentation is a secondary one ; but the limits of 
the secondary segments differ according to the relations of the sclerotiza- 
tion in the primary intersegmental regions to that of the segmental 
regions before and behind them. 

In the insect abdomen, where both the dorsal and the ventral primary 
intersegmental areas of sclerotization are continuous with the segmental 
plates following, a typical secondary segmentation prevails (Fig. 37, Ah), 
and the functional intersegmental rings are the membranous posterior 
parts of the primary segments (Fig. 40 A, Mh). The primitive inter- 
segmental fold {Isg) forms an antecosta (Ac) on both the tergum and 
the sternum, and the precostal lip forms an acrotergite (atg) of the dorsal 
plate and an acrosternite (ast) of the ventral plate. Wherever there is a 
difference between the dorsum and the venter in the sclerotization of the 
intersegmental region, however, the intersegmental relations are less 
simple. If the dorsal plates retain the form typical of secondary seg- 
mentation, while the ventral sclerotization takes the form of independent 



80 


PRINCIPLES OF INSECT MORPHOLOGY 


sternites and intersternites, as often occurs in the thorax, the ventral 
half of each intersegmental membrane will embrace the intersternite 
(B, 1st) and will include a part of the two adjoining primary segmental 
regions. Again, if the intersternite is united with the segmental sternum 
preceding (C), while the intertergal sclerotization remains continuous 
with the tergum following, the conjunctival membrane (iW6) will cross 
obliquely on the side of the body from the posterior part of the anterior 
segment to the anterior part of the posterior segment, and the ven- 
tral postcostal lip becomes a posisternite (pst) of the anterior segment. 
Finally, if both the dorsal and ventral intersegmental sclerotizations are 
united or closely associated with the segmental plates preceding, the 
functional intersegmental membrane becomes the anterior part of the 
posterior segment (D, Mb'), and the postcostal lip forms a posttergite 
iptg) dorsally and a poststernite (pst) ventrally. 



Fig. 40. — Four types of intersegmental relations, according to the position of the second 
ary intersegmental membrane, or conjunctiva (.Mb, Mb'). Ac, antecosta; acs, antecosta 
suture; as(, acrosternite; Isg, primary intersegmental line; ps(, poststernite; pig, posttergite 


Tagmosis. — In all adult arthropods some of the segments are more 
or less united into groups forming distinct trunk sections, or tagmata. 
The number of tagmata, and the number of segments in each tagma vary 
in different arthropods. The most constant tagmosis of the trunk is that 
which differentiates, in the embryo, the protocephalic head region from 
the primitive body (Fig. 23 A), The definitive head of most mandibulate 
arthropods, however, contains a second tagma, which is that of the 
gnathal segments (C, Gvi). In the Hexapoda a third embryonic tagma 
{.TK) becomes the thorax, or second tagma of the adult, which is usually 
composed of three segments, though in most H 3 anenoptera it contains 
four. A fourth embryonic tagma (A5) becomes the adult hexapod 
abdomen, which has at most 12 segments, including the periproct. 
Tagmosis is more variable in the Crustacea; in the Chilopoda and 
Diplopoda it results only in the formation of a head, including the 
^athal segments, and a body ; in the Chelicerata again it is variable, but 
the principal division of the trunk is into a “ cephalothorax ” and an 
“abdomen.” 




BODY REGIONS, SCLERITES, AND SEGMENTATION 


81 


GLOSSARY OF GENERAL TERMS APPLIED TO THE BODY SEGMENTS 
AND THE SKELETAL PLATES 

Acrosternite {asl). — The narrow marginal flange anterior to the antecosta of a 
definitive sternal plate that includes the preceding primary intcrsogmcntal sclerotiza- 
tion; characteristic of abdominal stoma of insects, but absent on thoracic sterna. 

Acrotergite (alg). — The anterior prccostal part of the tergal plate of a secondary 
segment; usually a narrow flange, but sometimes greatly enlarged, and frequently 
reduced or obliterated. 

Antecosta (Ac). — The anterior submarginal or marginal ridge on the inner sur- 
face of a tergal or sternal plate corre.sponding to the primary interscgmental fold, on 
which typically the longitudinal muscles are attached. 

Antecostal Suture (acs). — The external groove of the antecosta. 

Conjunctiva (Mb). — See inlersegmenlal membrane. (Gelenkhaut.) 

Dorso-pleural Line (a-a). — The line of separation between the dorsum and the 
pleural region of the body, often marked by a fold or groove. 

Dorsum (D). — The entire back of an animal above the pleural regions; or spe- 
cificall}', when qualified by the designation of a segment, the back region of a segment. 

Interscgmental Membrane (Mb). — The flexible conjunctiva between two sec- 
ondary segments; usually the nonsclerotized posterior part of a primary segment, but 
variable as shown in Fig. 40. 

Interstemite (Isl). — An intersegmental sclcrotization of the venter, such as the 
thoracic spinastema. 

Laterotergite (Itg). — A lateral sclerotization of the dorsum distinct from a principal 
median tergitc. (Paralcrgile.) 

Limb Basis (Cxpd). — The primitive ba.sal part of a limb, implanted in the pleural 
area of the body wall, bearing the telopodite. (Coxopodite.) 

Metamere. — An embryonic somite, or primary body segment. 

Myotome. — A division of the body muscles corresponding to a metamere. 

Notum. — See lergum. 

Paratergite. — See lalcrolergite. 

Pleural Region (P ). — The podial region, or ventrolateral parts of the body on 
which the limbs arc implanted, metamcrically divided into segmental pleural areas. 

Pleurite (pi). — Any minor sclcrite of the pleural area of a segment, or one of the 
component sclerltcs of a plcuron. 

Pleuron (PI). — The sclcrotization of the pleural area of a segment, apparently 
derived from the proximal part of the limb basis, and usually subdivided into pleurites. 

Pleuro-ventral Line (6-h). — The line of separation between the pleural region and 
the venter; lying mesad of the limb bases, but obscured when the latter arc fused with 
the sterna. 

Postnotum (PN). — An interscgmental plate of the dorsum of the thorax associ- 
ated with the tergum of the preceding segment, bearing the antecosta and usually a 
pair of phragmatal lobes. (Phragmanolum.) 

Poststemite (psl). — The postcostal lip of a definitive sternal plate that includes 
the intersegmental sclcrotization following. 

Posttergite (pig). — The narrow postcostal lip of a postnotal thoracic plate. 

Primary Segmentation. — A segmental division of the body corresponding to the 
embryonic metamerism. 

Sclerite. — Any sclerotized area of the body wall, or of internal parts derived from 
the body wall. 

Scleroma. — The sclerotic annulus of a body segment in distinction to the mem- 
branous conjunctiva. 



82 


PRINCIPLES OF INSECT MORPHOLOGY 


Secondary Segmentation. — Any form of body segmentation that does not strictly 
conform with the embryonic metamerism; the usual segmentation of arthropods hav- 
ing a well-developed exoskeleton, in which the membranous intersegmental rings are 
the posterior parts of the primary segments. 

Segment. — A body segment is any of the successive annular subdivisions of the 
arthropod trunk, whether corresponding to the embryonic metameres or produced 
secondarily. 

Somite. — A primitive, or primary, body segment corresponding to an embryonic 
metamere or myotome. 

Stemite. — A subdivision of a sternal plate, or any one of the sclerotic components 
of a definitive sternum. 

Sternum. — Either the primary ventral plate {Stn) of a body segment or a com- 
posite definitive sternum (S). 

Subcoxa (Sex ). — The proximal part of the limb basis when differentiated from 
the coxa; usually incorporated into the pleural wall of the body segment. 

Tagma. — A group jf successive segments forming a distinct section of the trunk. 

Tergite. — A subdivision of a definitive tergum, or any one of several sclerites in 
the dorsum of a body segment. 

Tergum (T ). — The dorsal sclerotization of a body segment; called also notum, 
especially in the thorax. 

Trtmk. — The entire series of body segments of an arthropod, including the cephalic, 
thoracic, and abdominal sections. 

Venter (F). — The entire under surface of the animal between the two series ol 
limb bases, or, when qualified by the designation of a segment, the corresponding sur- 
face of a single body segment. 



CHAPTER V 


THE SEGMENTAL APPENDAGES OF ARTHROPODS 

The Arthropoda are well named from the fact that they have jointed 
segmental appendages (from apdpov, a “joint,” and ttovs, ttoBos, a “foot” 
or “leg”), for no other feature of their organization is so characteristic 
of them. While segmented limbs are possessed by other groups of 
animals, they have nowhere attained such a diversity of form in adapta- 
tion to so wide a range of uses as have the appendages of the arthropods. 
Primarily the segmental appendages are organs of locomotion; but in 
the evolution of the Arthropoda they have developed into a great assort- 
ment of tools. The effective use of tools involves a high degree of 
efficiency in the muscular and nervous systems and the possession of 
varied and discriminating organs of perception. As a consequence, the 
arthropods are endowed vdth the highest mechanical, nervous, and 
sensory organization attained within the Invertebrata. 

General Structure of the Appendages. — The segmental appendages 
of arthropods are hollow outgrowths of the lateral or lateroventral 
regions of the body wall (Fig. 41). The early, perhaps wormlike, 
ancestors of the Arthropoda probably had a pair of appendages on each 
of the true somites between the prostomium and the periproct and may 
thus have resembled the onychophorans in general appearance. The 
appendages of modern arthropods are characteristically segmented, 
but the embryonic rudiments of the organs are simple lobes. We must 
assume, therefore, that the jointing of the limb is a secondary deA^elop- 
ment, and that the primitive appendage was an undivided lobe or tubular 
outgrowth of the body wall, serving as an aid in locomotion. 

The Basal Mechanism of a Primitive Appendage . — To be functionally 
effective, an organ of locomotion must be movable. There can be 
little question that the primitive arthropod appendages, whether used 
for progression on solid surfaces or in water, turned forward and rear- 
ward in the manner of an annelid parapodium, each on an approximately 
vertical axis of flexibility at its base (Fig. 41, a-h). A comparative study 
of the basal limb muscles in Annelida, Onychophora, and Arthropoda 
suggests that the simplest effective musculature of a primitive appendage 
comprises dorsal promotor and remotor muscles (J, J) arising on the 
dorsum of the body segment, and ventral promoters and remotors (K, L) 
arising on the venter. An actual musculature of this pattern is present 

?3 



84 


PRINCIPLES OF INSECT MORPHOLOGY 


in connection with all the locomotor appendages of Onychophora (Fig. 42, 
F), and with the simpler anterior parapodia of some Annelida (D), and 
occurs in a somewhat modified form in the entire series of annelid para- 
podia (E), while the basal limb musculature of arthropod appendages can 
in most cases be analyzed into the same functional groups of muscles. 
The relation of the limb muscles to the muscles of the body wall, how- 
ever, differs in these three groups of animals, so that it is evident there 
is no real homology involved in the similarity of the limb muscles; 
each case probably represents an independent structural adaptation to a 


DV DMcl 



Fig. 41. — Diagrammatic cross section of an arthropod segment showing the relation 
of the legs and the basal leg muscles to the areas of the body wall. a-b, axis of movement 
of leg base; Cxpd, coxopodite; DMd, dorsal longitudinal muscles; DV, dorsal blood vessel; 
I, J, dorsal promotor and remotor muscles of leg; K, L, ventral promotor and remoter 
muscles of leg; 0, levator muscle of telopodite; P, podial, or pleural, area of body wall; Q, 
depressor of telopodite; Stn, sternum; Tlpd, telopodite; VMd, ventral longitudinal muscles; 
VNC, ventral nerve cord. 

common function. We may not suppose, therefore, that the arthropod 
appendages have necessarily had a common origin with the parapodia of 
Annelida or with the tubular legs of Onychophora. 

Segmentation of the Appendages . — The limbs of the earliest known 
fossil arthropods are fully segmented; the legs of the Cambrian trilobites 
and crustaceans have all the segments that occur in modern arthropods. 
Embryology throws little light on the evolution of the arthropod append- 
ages, and we can deduce a working hypothesis as to the homologies of 
the limb segments only from a comparative study of the structure of 
the segments in the several arthropod groups, and from a theoretical 
consideration of the mechanical demands in an organ of locomotion. 

An appendage having the form of a hollow outgrowth of the body 
wall and containing an extension of the body muscles is capable of seg- 
mentation in the same way as the cylindrical body itself, namely, by 
the attachment of its muscles to successive parts of its wall. An incipient 
limb segmentation is to be seen in the onychophoran leg (Fig. 42 H), in 
the distal part of which are several distinct annuli separated by infoldings 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


85 


of the integument, on some of which muscles are attached (15, 16, 19), 
but there is nothing here closely resembling the definite relation between 
muscles and segments so characteristic of an arthropod limb. 

A clearly defined limb segment, definitely correlated with muscle 
attachments, is termed a podomere, or podite. In the typical arthropods 
the limb segmentation is so thoroughly standardized that the number of 



Fig. 42.' -Appendages of Annelida and Onychophora. A, B, C, parapodia of Nereis 
Virens. D, muscles of an anterior parapodium. E, lateral muscles of a typical segment of 
Nereis, showing muscles (/, J, j, K, L) attached to base of parapodium. F, musculature 
of right half of a body “ segment” of Peripatoides, showing four muscles (/, J, K, L) entering 
base of leg, Gi cross section of leg of Peripatoides. H, longitudinal section of same. 

segments seldom if ever exceeds eight; but with specialization there is 
often a union of two consecutive primitive segments, accompanied by the 
loss of the muscles of the more distal segment, or segments may be reduced 
or obliterated. On the other hand, a segment may become secondarily 
divided into two or more nonmusculated subsegments. A true limb 
segment, or podite, therefore, must be defined as any part of an appendage 
independently movable in some member of the Arthropoda by muscles 
inserted on its base. The areas of flection between the podites are the 
joints of the limb; particular sclerotic points of contact in the joints are 
articulations. 


86 


PRINCIPLES OF INSECT MORPHOLOGY 


The names used generally in zoology to designate the arthropod 
leg segments, beginning at the base of the appendage, are as follows; 
coxopodite, basipodite, ischiopodite, meropodite, carpopodite, propodite, 
dactylopodite. In entomology the following terms are more commonly 
used for the same segments: coxa, first trochanter, second trochanter (or 
prefemur), femur, tibia, tarsus, pretarsus. In some of the Chelicerata 
an extra segment, the patella, is interpolated between the femur (meropo- 
dite) and the tibia (carpopodite). Strictly speaking, the term “coxa” 
refers to a distal subdivision of the coxopodite when the latter is divided 
into a subcoxa and a coxa. 



Fig. 43. — Diagrams of the evolution of the segmentation of an arthropod leg. A, 
theoretically primitive limb divided into coxopodite (Cxpd) and telopodite {Tlpd). B, the 
telopodite segmented at the femoro-tibial joint (/i). C, a primitive insect leg, with coxo- 
podite divided into subcoxa (Sex) and coxa (Cx), the telopodite six-segmented. D, a 
typical arachnid limb with a patella (JPal) interpolated between femur and tibia. 


In an elongate appendage turning forward and rearward on its base, 
the first demand for a point of movement in the shaft would establish a 
joint near the base allowing movement of the distal part in a vertical 
plane. Thus we may assume that the primitive arthropod limb first 
became divided into a basis, which is the coxopodite (Fig. 43 A, Cxpd), 
and a distal arm, or telopodite {Tlpd). 

Further evolution toward mechanical efl&ciency in an elongate 
appendage evidently would result in the production of a “knee” joint 
in the telopodite, giving the part beyond the knee a principal down- 
ward flection in a vertical plane (Fig. 43 B). Hence there have come to 
be two primary points of bending in the limb, which persist in all typical 
arthropod appendages as the coxo-trochanteral joint (ct), and the femoro- 
tibial joint (ft). By a further segmentation, the proximal piece of the 
telopodite may have one or two trochanters (C, ITr, 2Tr) cut off from its 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


87 


base, leaving the rest as the femur (Fm), while the part of the limb 
distal to the knee becomes divided into a tibia {Tb), a tarsus {Tar), 
and a pretarsus {Ptar). This type of limb segmentation is characteristic 
of all the mandibulate arthropods. In many of the Chelicerata there is in 
some of the appendages an additional segment, the patella (D, Pat), inter- 
polated between the femur and the tibia. The patella and the tibia 
together in this type of limb segmentation apparently represent the tibia 
alone of the Mandibulata (C, Th). In the Arachnida, Pycnogonida, 
Chdopoda, and Insecta, the tarsus is commonly divided into movable 
subsegments ; but since these parts are not provided with muscles, they are 
evidently secondary and are not to be regarded as true podites; they are 
tarsites, or tarsomeres. 



Fig. 44. — Comparative structure of a theoretically generalized arthropod limb (A), a 
trilobite leg (B), and a crustacean leg (C). Bnd, coxal endite; Bspd, basipodite; Cxpd, 
coxopodite; Endpd, endopodite; Endt, endites; Evpd, epipodite; Expd, exopodite; Ext, 
exites; Tlpd, telopodite. 

Lobes of the Appendages. — Along the outer and inner margins of 
an appendage there may be developed movable lobes often individually 
provided with muscles arising in the shaft of the limb. An outer lobe 
is known as an exite (Fig. 44 A, Ext), an inner lobe as an endite {Endt). 
Usually there is not more than one lobe in each position on a single 
segment, though sometimes two occur. 

The Limb Basis, or Coxopodite. — ^The basal segment of a generalized 
arthropod appendage is implanted in the membranous pleural wall 
of the body segment and may be articulated to the tergum or the sternum 
or to both. Upon its proximal margin are inserted the muscles that 
move the appendage as a whole, which take their origins on the dorsal 
and ventral walls of the body segment. A limb basis, or coxopodite, 
having this relatively primitive structure appears to be preserved in the 
limbs of the Trilobita, Xiphosura, many of the Arachnida, and most 
of the Crustacea. 

In some cases the limb basis loses its mobility and becomes merely 
a support for the rest of the appendage by assuming the form of a lobe 



88 


PRINCIPLES OF INSECT MORPHOLOGY 


or plate of the body wall. When this occurs, the basal muscles are 
reduced or suppressed, and the muscles of the first trochanter, arising 
in the basis, become the motors of the free part of the limb, which is 
the telopodite. 

In most cases, however, when the basis forms an immovable or but 
slightly movable support for the rest of the limb, it becomes subdivided 
into a proximal stationary part, or subcoxa (Fig. 43 C, Sex), and a distal 
freely movable part, or coxa (Cx). The subcoxa is usually incorporated 
into the pleural wall of the body segment, where it forms a group of 
sclerites or a plate known as the pleuron. The coxa now becomes the 
functional basis of the appendage. 

The subcoxae appear in a relatively primitive condition in some 
of the Chilopoda, where their sclerotized parts may form complete 
rings about the bases of the coxae, though more commonly each is broken 
up into several small sclerites lying in the pleural wall of the body 
segment close to the coxa (Fig. 52 A, Sex). In the insects the pleural 
plates of the thoracic segments appear also to be derived from the sub- 
coxal parts of the leg bases. Those of the Apterygota (Fig. 89) consist of 
small sclerites as in the Chilopoda, but in adult Pterygota the pleura are 
extensive plates in the lateral walls of the thoracic segments, which, 
in the second and third segments, are extended upward to form supports 
for the wing bases (Fig. 91 B). In the decapod Crustaceans subcoxal 
extensions from the bases of the pereiopods on each side of the body are 
united in a large pleural plate forming the inner wall of the branchial 
chamber. The ventral arcs of the subcoxae are generally reduced to 
narrow folds between the coxae and the sterna, or they unite with the 
primary sterna to become laterosternal elements of the definitive sternal 
plates. 

The coxa and subcoxa of an appendage never have in all respects 
the structural relations of true primary segments to each other. The 
body muscles of the limb are usually inserted on both the subcoxa and 
the coxa or are taken over entirely by the coxa. The coxa, however, 
generally is provided also with muscles arising in the subcoxa. The 
subcoxo-coxal articulation is variable, though it appears that the primary 
hinge of the coxa on the subcoxa was approximately in a vertical plane 
with dorsal and ventral points of articulation. 

Exite and endite lobes are of frequent occurrence on the appendage 
basis. In the Trilobita and some of the phyllopod Crustacea basendites 
are present on the majority of the appendages (Figs. 45, 50 B, End), form- 
ing a double row of lobes converging along the midventral line of the 
body. Basendites of the gnathal appendages function as feeding acces- 
sories. The maxillae of crustaceans and insects usually have each a pair 
of basendites, known as the lacinia and galena. The diplopods have a 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


89 


freely movable mandibular endite, but in the chilopods, crustaceans, and 
insects the mandibular endite is consolidated with the basis of the 
appendage to form a solid jawlike organ. Basendites are of less frequent 
occurrence in the Chelicerata, but they are often present on the pedipalps 
and other appendages associated with the mouth. 

An exite, or outer lobe, of the coxopodite is commonly termed an 
epipodite and is often a highly developed or specially modified structure 
(Fig. 44 B, C, Eppd). The epipodite is an important feature of all the 
legs of Trilobita and of some of the limbs of many Crustacea, since in both 
these groups it may be converted into a branchial organ (Fig. 45). 
Gill-bearing epipodites are present also on the abdominal appendages 
of Xiphosura. The epipodite of Crustacea, however, is often a simple 
lobe or is sometimes represented by a pair of lobes (Fig. 50 A, Eppds) 
and may be absent. Epipodites seldom occur on the appendages of 
terrestrial arthropods, though the appendicular processes known as 
styli, present on the coxae of the second and third legs of the apterygote 
insect Machilis (Fig. 148 A, Sty), are of the nature of epipodites. 

The Telopodite. — The distal shaft of the limb, or telopodite, is highly 
variable in size and segmentation since it takes on numerous forms in 
adaptation to different functions, such as walking, running, leaping, 
climbing, grasping, or swimming, and it may be rudimentary or sup- 
pressed; in the gnathal appendages it becomes the “palpus.” The 
identity of the telopodite, however, is seldom to be mistaken. Except in 
rudimentary appendages and in some crustacean limbs in which the 
basipodite is united with the coxopodite, the telopodite almost universally 
articulates with the coxopodite on a horizontal hinge with anterior and 
posterior articular points. Allowance must be made, of course, for an 
axial revolution of the limb, which may actually lie in an oblique or even a 
horizontal plane. The basal muscles of the telopodite are the levator and 
depressor of the first trochanter (basipodite), which arise in the coxopodite 
(Fig. 43 A, 0, Q), except that in insects the levator of the legs usually 
has one or more branches arising in the body segment. Elevation 
and depression of all the appendages in an entire lateral series thus 
take place uniformly along the line of the coxo-trochanteral joints, 
except, as in some Crustacea, where the first trochanters are united with 
the coxae. 

Endite and exite lobes are of frequent occurrence on the segments 
of the telopodite in the Crustacea; and in this group an exite of the 
basipodite (first trochanter) is of particular importance, since it is often 
highly developed as an accessory outer branch of the limb called the 
exopodite (Figs. 44 C, 50 A, Bxpd). The crustacean limb thus acquires 
its characteristic “biramous” structure, the shaft of the telopodite 
beyond the basipodite being known as the endopodite (Endpd). It 



90 


PRINCIPLES OF INSECT MORPHOLOGY 


should be observed that a branched limb of this type occurs only in 
the Crustacea, and that there is no sound evidence of its being the 
primitive limb form of arthropods generally. 

The maximum number of segments in the telopodite appears 
to be six in Mandibulata and seven in Chelicerata, with six as the usual 
number in Trilobita. The number may be variously reduced, however, 
either by a union of successive segments or by suppression of certain 
segments. The second trochanter (ischiopodite) is perhaps the most 
variable segment. In some forms it is not differentiated from the 
femur (meropodite) in all the appendages ; in the legs of most Hexapoda 
it is united with the first trochanter (basipodite) in a single trochanteral 
segment (Fig. 53 A, Tr). The tarsus is variable in its subsegmentation. 
In Chelicerata and Chilopoda it is frequently divided into two sub- 
segments, a basitarsus and a telotarsus (Figs. 48 D, 49, 52), and sometimes 
it is further broken up into a large number of small articles (Fig. 47 A). 
Among the Hexapoda the tarsus may be a simple segment, or it may be 
divided into from two to a maximum of five subsegments. The pretarsus 
(dactylopodite) is typically a simple clawlike segment; but it may be 
armed with a pair of lateral claws, as in some Arachnida, Pycnogonida, 
and most insects, in which case the median claw is usually reduced or 
obliterated, and the pretarsus becomes secondarily a two-clawed 
structure. 

The joints of the telopodite usually have a characteristic movement. 
The trochanteral and trochantero-femoral joints commonly have a 
movement of production and reduction; the femoro-tibial joint has a 
downward flexure, and the tibio-tarsal and tarso-pretarsal joints move 
likewise in the axial plane of the limb. The patello-tibial joint, when 
present, is variable in respect to the direction of its axis, but it generally 
gives a movement of production and reduction to the tibia. The limb 
joints are monocondylic or dicondylic according to whether they contain 
one or two points of contact between the adjacent segments. The 
articulation in a monocondylic joint is usually dorsal; in a dicondylic 
joint the articular points are commonly anterior and posterior, though 
they may be dorsal and ventral. 

Legs of Trilobita. ^The trilobites present the most generalized 
condition of the appendages found in the Arthropoda, since limbs occur 
on all the body segments but the last, and they are all practically alike 
in form. The first pair of appendages arises behind the base of the labrum 
(hypostoma) ; they are possibly procephalic antennae, since they are usu- 
ally long, simple, and multiarticulate. The rest of the appendages are leg- 
like in structure and are undoubtedly postoral limbs. Each consists of a 
large basal segment, or coxopodite (Fig. 45, Cxpd), and of a slender telo- 
podite (Tlpd) containing usually six segments. The coxopodites have 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


91 


large endite processes (Bnd) directed mesally, and, except some of the 
more anterior appendages, each supports a long fringed epipodite (Bppd), 
which is probably, as shown by Stormer (1933), a branchial organ. The 
trilobite appendages are often described as being biramous, but it should 
be observed that the outer branch is an epipodite of the coxopodite and 
therefore does not represent the exopodite of biramous crustacean limbs. 



Fig. 45.- — Diagrammatic cross section of a trilobite. 


Legs of Xiphosura. — The first three of the four pairs of legs of Xipho- 
sura polyphemus have only six distinct segments; the last leg (Fig. 46), 
however, has seven. The large first segment of each appendage is the 
coxopodite (Cxpd), which in the hind leg bears dorsally a spatulate 
epipodite (Eppd). The next segment is evidently a trochanter (Tr), 
and the third the femur (Fm). The fourth segment of the last leg appears 



Fig. 46. — ^Last leg of Xiphosura polyphemus^ left side, anterior view, 

to be a patella (Pat), since it is followed by a tibia (Tb), a tarsus (Tar), 
and a pretarsus (Ptar). In the other legs, which have one less segment, 
either the patella or the tibia is lacking. The musculature suggests that 
the missing segment is the tibia. The presence of a patella gives the 
xiphosuran leg a resemblance to the leg of an arachnid, especially to that 
of a scorpion (Fig. 48 D) ; but the origin of the levator and depressor 
muscle of the pretarsus in the tarsus (Fig. 46) is a primitive character 
found in Crustacea and Pycnogonida, and not in Arachnida. The abdom- 



92 


PRINCIPLES OF INSECT MORPHOLOGY 


inal appendages of Xiphosura are reduced and have the form, of wide 
transverse plates. Each consists of a large coxopodite supporting a 
broad, flat epipodite and of two or three small distal segments. 

Legs of Arachnida. — The arachnid limb, in its fullest segmentation, 
shown in the leg of a solpugid (Fig. 48 B), a tick (Fig. 47 C), or a phal- 
angid (Fig. 47 A), consists of a coxopodite, two trochanters, a femur, 
a patella, a tibia, a tarsus, and a pretarsus. 

The coxopodite is never subdivided into a subcoxa and a coxa. It is 
inserted in the lateral or ventrolateral wall of the body (Fig. 47 A), where, 
if movable, it turns forward and rearward and may be articulated to the 
tergum if there are well-developed plates in the body wall. When 



Fig. 47. — Appendages of Arachnida, A, second right leg of a phalangid, Liohunum. 

B, left pedipalp of Liohunum. C, fourth leg of a tick, Amhlyomma tuberculatum. 

movable, the basis is provided typically with dorsal (7, J) and ventral 
{K, L) promotor and remotor muscles, though one set may be lacking. 
The dorsal muscles have their origin on the tergum; the ventral muscles 
arise on the sternum or on sternal apophyses, or on a transverse ligament 
suspended in the body, which probably is derived from the sternum. 

The segments of the arachnid telopodite are variable in different 
appendages and in corresponding appendages in the several arachnid 
groups. Two well-defined trochanters are present in some of the legs of 
Solpugida (Fig. 48 B, l^r, 2Tr), and an indistinct second trochanter, 
or prefemur, appears to be only partially separated from the base of the 
femur in phalangids (Fig. 47 A) and ticks (C). The legs of pseudo- 
scorpions, scorpions (Fig. 48 D), and spiders, however, have only one 
trochanter, and two trochanters are never present in the pedipalp (Fig. 
47 B). The patella is absent in the pseudoscorpions. The tarsus is 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


93 


often divided into two subsegments (Fig. 48 D), either one or both of 
which may be further subdivided (Figs. 47 A, 48 C). The pretarsus is a 
single claw in the phalangids and has a simple, dactylopodite-like form 
in the primitive genus Holosiro. In most of the arachnids, however, the 
pretarsus bears two lateral claws (Fig. 48 E, Un) and is itself reduced to 
a median hook or spur (Ptar), though it retains the dicondylic hinge (r) 
with the tarsus and has both levator and depressor muscles (Iptar, dptar). 
The pretarsal muscles in all Arachnida arise in segments proximal to the 
tarsus. 




Pig. 48. — ^Lega of Arachnida. A, B, second and third legs of a solpugid. C, distal 
part of a solpugid leg with segmented tarsus. D, leg of a scorpion. E, terminal parts of 
a scorpion leg, showing reduced pretarsus (Pfar), with apodemes of levator and depressor 
muscles and lateral pretarsal claws (Uri). 

Legs of Pycnogonida. — The pycnogonid leg (Fig. 49 A) resembles the 
leg of an arachnid in having a patella (Pat) interpolated between the 
femur and the tibia. The distinctive feature of the pycnogonid append- 
age is in its proximal part, where there are three small segments inter- 
vening between the femur (Fm) and a supporting lobe (L) of the body 
segment. At first glance these segments might appear to be the coxopo- 
dite and the usual two trochanters, but their articulations are not typical 
of these segments. The horizontal dicondylic hinge of the first segment 
on the body lobe, or that between the second and third segments, suggests 



94 


PRINCIPLES OF INSECT MORPHOLOGY 


the characteristic coxo-trochanteral joint of other arthropods, while a 
vertical hinge such as that between the first and second segments never 
occurs elsewhere between the coxa and trochanter. Some writers regard 
the body lobe as the limb basis or as a subcoxal limb segment united with 
the body. 

The pretarsus of the pycnogonids is a small dactylopodite with levator 
and depressor muscles arising, as in Xiphosura and Crustacea, in the 
tarsus (Fig. 49 B), which in Pycnogonida is divided into two subsegments. 
In some species there is a pair of small accessory claws {B, Un) arising 
dorsally from the base of the pretarsus. 



Fig. 49. — Leg structure of Pycnogonida. A, third right leg of Chaetonymphon 
spinosum and attachment to body. B, distal part of leg, showing levator and depressor 
muscles of pretarsus arising in tarsus, and small pretarsal claws (Un). 

Legs of Crustacea. — A typical crustacean appendage has the usual 
seven limb segments of the Mandibulata (Fig. 50 A). The coxopodite is 
generally undivided, but in the Decapoda the gill-bearing plates forming 
the inner walls of the branchial chambers appear to be expansions of the 
subcoxal parts of the bases of the ambulatory legs. Ordinarily the gills 
are modified epipodites of the coxopodites or filamentous structures 
borne on the epipodites. The segments of the telopodite may be vari- 
ously modified, reduced, or eliminated, and in simplified appendages the 
basipodite is sometimes united with the coxopodite, for min g a composite 
limb base termed the protopodite. The dactylopodite is usually a simple 
clawlike segment, though it may be opposed by a process of the propodite, 
forming a chela. In some of the Isopoda, however, the dactylopodite 
bears a pair of small claws on its base similar to the lateral claws of insects 
and some arachnids. The dactylopodite is provided with levator and 
depressor muscles, which arise in the propodite (tarsus) . 

A distinctive feature of crustacean appendages in general, though one 
by no means always present, is the special development of an exite lobe 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


95 


on the basipodite into a long, often jointed arm known as the exopodite 
(Fig. 50 A, Expd). The presence of the exopodite gives the limb its 
so-called biramous structure, the inner arm, or endopodite, being the 
shaft of the telopodite distal to the basipodite. In some cases the exo- 
podite exceeds the endopodite in size; or the endopodite may be sup- 
pressed, resulting in a monoramous limb of which the distal part is the 
exopodite. In most of the Crustacea some of the appendages are modi- 
fied in structure for purposes of swimming. In certain groups the swim- 
ming organs are large, flat exite lobes (Fig. 50 B, Ext ) ; in others the 
exopodite and endopodite branches of a group of appendages set apart as 
natatory organs take on the form of broad overlapping plates. 



Fig. 60. — Appendages of Crustacea. A, fourth right pereiopod of Anaspides tasmaniae. 

B, third maxilliped of Apus longicaudata. 

Legs of Diplopoda and Symphyla. — In the diplopods the legs arise 
from the ventral plates of the body segments, and it is impossible to 
determine in the adult whether subcoxal parts of the limb bases are 
incorporated in these plates are not. The first segment of the leg (Fig. 
51, A, Cx), however, is evidently the homologue of the coxa in the 
Chilopoda. The second segment, then, is the first trochanter (ITr), 
the third, though from its size it may have the appearance of a femur, is 
probably the second trochanter {2Tr), and the small fourth segment the 
femur (Fm). Distal to the femur are the usual tibia (Th), tarsus {Tar), 
and pretarsus (Ptar). The pretarsus has a single muscle, the depressor 
(A, B, 13), arising in the tibia, which is inserted on the base of the pre- 
tarsus by a long tendinous apodeme (x) traversiog the tarsus. The 
pretarsal musculature of the diplopods thus corresponds to that of the 
chilopods and hexapods, in which a levator of the pretarsus is absent, 
and the depressor arises proximal to the tarsus (Figs. 52 B, 53). 

The legs of Symphyla (Fig. 51 C), except those of the small fia-st pair, 
closely resemble the diplopod legs both in segmentation and in the 



96 


PRINCIPLES OF INSECT MORPHOLOGY 


relative size of the second trochanter and the femur. Each leg carries 
a small styluslike basal process (Bnd), which, in Hanseniella at least, is an 
endite of the coxa and is therefore not comparable with the thoracic 
styli of Machilidae (Fig. 148 A, Sty), which are coxal exites. The claw- 
like pretarsus has a small posterior claw arising from its base (Fig. 51 
C, Ptar). The pretarsal musculature, according to H. E. Ewing (1928), 
consists of a depressor muscle only, the fibers of which, as in Diplopoda, 
arise in the tibia. 

In the Pauropoda the legs have only six segments, there being but a 
single trochanter. The pretarsus lacks a levator muscle, and the fibers 
of the depressor arise in the tibia and femur. In certain forms the tarsus 
is divided into two or three subsegments. 



Fig. 61. — ^Legs of progoneate Myriapoda. A, left leg of a diplopod, Euryurus, 
anterior view. B, distal segments of same, posterior view. C, right leg of a symphylid, 
Hansenidla, 

Legs of Chilopoda. — ^The legs of the chilopods are implanted in broad 
membranous pleural areas of the body segments, in which there is always 
a distinct and more or less sclerotized fold surrounding the base of the 
coxa, that evidently represents a subcoxal part of the limb basis (Fig. 
52 A, Sex). The dorsal muscles of the appendage are inserted on the 
subcoxa, and the coxa turns on the subcoxa by a vertical hinge with 
dorsal and ventral articular points (c, d). The subcoxal sclerotization is 
continuous around the base of the coxa in some Geophilidae, but in most 
of the other chilopods it is broken up into one or more small sclerites 
tfix-pl, Spl). Two trochanters are always present, which in Geophilidae 
are movably articulated to each other. In Lithdbius and Scolopendra 
they are united, and if the leg is broken off in these forms it comes free 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


97 


at the coxotrochanteral joint; in Scutigcra the break occurs between the 
two trochanters. The tarsus {Tar) is usually divided into two subseg- 
ments; in Scutigcra each subsegment is again subdivided into a large 
number of small articles. The pretarsus (Plar) is a small dactylopodite- 
like claw; it is provided with a depressor muscle only, the fibers of which 



Fio. 52 . — ^Lor Btructurc of Chilopoda. A, IcR nnd loft sido of body segment of 
lAthohhis, Bhowing subcoxn (Scj) incorporated into pleural area of body -wall. B, distal 
part of log, showing single muscle (depressor) of pretarsus, n-ith branches arising in tibia 
and femur. 

arise in the tibia and the femur (B, dptar) and are inserted by a long 
tendon (x) on the ventral edge of the base of the pretarsus. 

Legs of Hexapoda. — In the proturans and insects the free part of the 
leg comprises at most but six independently movable segments (Fig. 



Fig. 63. — ^Legs of Hexapoda. A, third leg of Eosentomon gcrmanicum. {From 
Prdl, 1912.) B, third right leg of a caterpillar, Estigmene acraea, posterior view, coxa 
removed. 

53 A), namely, a coxa, one trochanter, a femur, a tibia, a tarsus, and a 
pretarsus. In the Odonata there are two trochanters, though they are 
not movable on each other. The second contains a redactor muscle of 
the femur. The femur is usually the largest segment of the leg. Beyond 
the knee joint the segmentation is variable, and there is often a reduction 



98 


PRINCIPLES OF INSECT MORPHOLOGY 


in the number of segments, resulting either from a fusion of adjacent 
segments or from the obliteration of a segment. In many such cases a 
careful study of the leg musculature will help establish the identity of the 
parts present. The tarsus may be an undivided segment, or it may be 
broken up into two, three, four, or five subsegments; but it is not charac- 
teristically divided into a basitarsus and telotarsus as in the Chilopoda 
and some Arachnida. No muscles have their origins within the tarsus 
in insects. 

The hexapod pretarsus in its simplest form consists of a small clawlike 
segment (Fig. 53, Ptar) similar to the terminal claw of a chilopod or diplo- 
pod limb, and, as in these two groups, it is provided vuth a depressor 
muscle only. This muscle arises usually by several branches distributed 
in the tibia and the femur (dpiar), which are inserted on a long slender 
apodeme or “tendon” (x) that traverses the tarsus to its attachment on 
the ventral lip of the base of the pretarsus. The usual pretarsus of adult 
insects comprises a pair of lateral claws, the ungues, articulated dorsally 
to the end of the tarsus (Fig. Ill A, Un), and a median structure {Ar) 
which is probably a remnant of the primary dactylopodite. A condition 
intermediate between the one-clawed and two-clawed types of structure 
is found in certain Collembola that have a pair of small lateral claws 
developed from the base of a larger median claw, and in some of the 
Thysanura where there are two articulated lateral claws (Fig. 110 C, D), 
and a small median claw (doc), to the base of which is attached the tendon 
(x) of the depressor muscle. In adult pterygote insects the tendon of the 
depressor muscle (“retractor of the claws”) is usually attached to a small 
ventral sclerite in the base of the pretarsus (Fig. Ill C, E, Utr). The 
lateral claws iJJn) are clearly secondary structures developed dorsally 
from the base of the pretarsus. 

GLOSSARY OF TERMS APPLIED TO THE APPENDAGES 

Arolium (Ar ). — The median terminal lobe of an insect’s foot, probably a remnant, 
of the dactylopodite. 

Basipodite (JSspd). — ^The basal segment of the telopodite, or second segment of a 
generalized appendage; in Crustacea bearing the exopodite. {First trochanter.) 

Basitarsus {Btar ). — The proximal subsegment of the tarsus. 

Carpopodite (Crpd ). — ^The fifth segment of a generalized appendage. {Tibia.) 

Coxa {Cx ). — The distal part of the coxopodite serving as the functional basal seg- 
ment of the leg when separated from the proximal part (subcoxa). 

Coxopodite (Cxpd ). — The primary basal segment of an appendage, representing 
the primitive limb basis. 

Dactylopodite {Dac ). — The terminal segment of a generalized appendage; typically 
clawlike in form, with levator and depressor muscles arising in the propodite; repre- 
sented by a median claw of the pretarsus in some apterygote insects. 

Endite. — A mesal lobe of any limb segment. 

Endopodite {Endpd ). — The mesal branch of a biramous appendage; the main shaft 
of the limb beyond the basipodite. 



THE SEGMENTAL APPENDAGES OF ARTHROPODS 


99 


Epipodite (Eppd). — An exite of the coxopodite; often a gill-bearing organ. 

Exite. — An outer lobe of any limb segment. 

Exopodite {Expd). — An exite of the basipodite in Crustacea, often highly devel- 
oped, giving the limb a biramous structure. 

Femur {Fm). — The third segment of the telopodite, usually the principal segment 
of the insect leg. {Meropodile.) 

First Trochanter (ITr). — The first segment of the telopodite. (Basipodite.) 

Ischiopodite. — ^The third segment of a generalized limb, or second segment of the 
telopodite. (Second trochanter, prefemur.) 

Limb Basis (Cxpd). — The primary basal segment of an appendage (coxopodite) 
supporting the telopodite; sometimes subdivided into a proximal subcoxa (pleuropo- 
dite, or pleuron), and a distal coxa. 

Meropodite. — ^The fourth segment of a generahzed limb. (Femur.) 

Patella (Pat). — A segment between the meropodite (femur) and the carpopodite 
(tibia) in the legs of Pycnogonida, most Arachnida, and in the last legs of Xiphosura. 

Podite, or Podomere. — A limb segment. 

Pretarsus (Ptar). — The terminal hmb segment (dactylopodite); in insects compris- 
ing usually a pair of lateral claws (ungues), and reduced median parts (arolium, ungui- 
iractor plate, or median claw). 

Propodite. — The penultimate segment of a generalized limb. (Tarsus.) 

Protopodite. — The basal stalk of some crustacean limbs composed of the united 
coxopodite and basipodite. 

Second Trochanter (2Tr). — The second segment of the telopodite, often not 
distinct from the base of the femur; in insects usually fused with the first trochanter. 
(Prefemur, ischiopodite.) 

Subcoxa (Sex). — A secondary proximal subdivision of the coxopodite present in 
some arthropods, forming a support in the pleural wall of the body segment for the 
rest of the appendage. (Pleuron.) 

Tarsomere, or Tarsite. — One of the subsegments of the tarsus. 

Tarsus (Tar.) — ^The penultimate segment of the limb, commonly divided into two 
principal subsegments or into a number of small parts, none of which is individually 
provided with muscles except the first. (Propodite.) 

Telopodite (Tlpd). — The primary shaft of the limb distal to the coxopodite, the 
basal segment of which is the first trochanter (basipodite). 

Telotarsus. — The distal of the two principal subsegments of the tarsus in Arach- 
lida and Chilopoda. 

Tibia (Tb). — The fourth segment of the telopodite in an appendage lacking a 
patella. (Carpopodite.) 

Trochanters (Tr). — The first and second segments of the telopodite (basipodite 
and ischiopodite)-, in insects generally united in a single trochanteral segment. 

Ungues (Un). — ^Lateral claws of the foot secondarily developed from the base of 
the dactylopodite; characteristic of insects, but occurring in several arthropod groups. 

Unguitractor Plate (Utr). — A ventral sclerite in the base of the pretarsus of insects 
upon which the depressor muscle of the pretarsus, or retractor of the claws, is inserted 
by a long tendinous apodeme. 



CHAPTER VI 


THE HEAD 

The head of an elongate animal is the compact anterior end of the 
trunk in which are crowded the principal instruments necessary to the 
creature for finding its way about in its medium, and usually the imple- 
ments essential to it for procuring and swallowing its food. The head is 
always at the anterior end of the body because the original direction 
of movement determined the pole at which cephalization should take 
place. Habitual progression in one direction made it necessary that the 
guiding sense organs should be located at the forward pole and also made 
this extremity of the animal the most practical location for the intake 
orifice of the alimentary canal. With the principal sense organs and the 
mouth located in the head, it follows that the head should contain the 
major sensory ganglia and should bear the organs for grasping and mani- 
pulating the food. Thus it comes about that in the head are associated 
the two extremes of animal activity, the highest mental powers, and the 
most primitive function of ingestion. 

1. GENERAL MORPHOLOGY OF THE ARTHROPOD HEAD 

In the evolutionary history of the arthropods the first well-defined 
head must have been an anterior section of the trunk corresponding to 
the region of the procephalic lobes of the embryo (Fig. 23, Prc). Con- 
sidered phylogenetically, therefore, we may term this primitive head the 
protocephalon. The protocephalon, as represented by the procephalic 
lobes of the embryo, consists of the circumoral region bearing the 
labrum, the eyes, and the antennae and usually includes the postoral 
somite of the second antennae, though the latter may be a distinct 
segmental region of the trunk immediately behind the cephalic lobes. 
The protocephalon persists in some Crustacea as a small definitive head 
carrying the labrum, the eyes, and both pairs of antennae, but in most of 
the mandibulate arthropods the adult head is a more complex structure 
(syncephalon) including the protocephalon and a variable number of 
succeeding somites, all more or less intimately united. Since the append- 
ages of the added somites are transformed into organs of feeding, these 
somites are known as the gnathal segments. The embryonic gnathal 
segments constitute a distinct gnathal section of the trunk (Fig. 23 C, 
Gn) between the protocephalon {Prc) and the thorax {Th). Inasmuch as 

100 



THE HEAD 


591.4 

S52P 


the definitive arthropod head of the composite type is thus formed by the 
union of two primitive sections of the trunk, we may distinguish in its 
composition a proto cephalic region, or procephalon, and a gnathal region, 
or gnathocephalon. An understanding of the morphology of the arthropod 
head depends largely on a study of the head appendages and the cephalic 
nervous system. 

The Procephalon. — ^The region of the procephalic lobes in the arthro- 
pod embryo bears the mouth, the labrum, the eyes, and at most three 
pairs of appendage rudiments. There is no doubt concerning the exist- 
ence of two pairs of procephalic appendages, namely, the antennae and 
postantennae (second antennae or ehelicerae), but the claim that there are 
three is based only on the presence of two small, evanescent preantennal 
lobes that have been observed to arise at the sides of the mouth in young 
embryos of a centipede and of a phasmid insect (Fig. 70 A, B, Prnt). 
The antennal and postantennal appendages (Fig. 23 B, Ant, Pnt) may 
also be suppressed in the adult, but one or both pairs of them are usually 
retained and variously developed in different arthropod groups. 

The procephalic region of the embryo shows no clear external segmen- 
tation, but it is said that its mesoderm in lower insects contains three 
pairs of coelomic sacs corresponding to the preantennal, antennal, and 
postantennal appendages. This, together with the triple division of the 
brain into protocerebral, deutocerebral, and tritocerebral lobes, is usually 
taken as evidence of a corresponding metamerism in the procephalon. 
It is necessary to assume, however, that the procephalon contains also a 
prostomial element (acron), and that the protocerebral lobes of the brain 
are largely made up of the primitive prostomial ganglion, or archicere- 
brum, which innervates the eyes. The antennae are the deutocerebral 
appendages, and the postantennae are the tritocerebral appendages. 

The general structure of the arthropod head and the cephalic nervous 
system is not entirely in accord with the foregoing theory of the procephalic 
segmentation. The mouth appears to lie immediately before the 
tritocerebral somite, and the first ventral dilator muscles of the 
stomodaeum are said by Smreczynski (1932) to be formed from the meso- 
derm of this somite. The tritocerebral ganglia, though united with the 
brain, are the first ganglia of the ventral nerve cord, and they innervate 
the oral and ventral preoral region of the head. The protocerebral and 
deutocerebral lobes of the brain, on the other hand, are always suprasto- 
modaeal in position and thus appear to be preoral nerve centers, but there 
is no evidence whatever of cephalic segmentation between the labrum and 
the mouth. A comparative study of the histology of the arthropod brain 
made by Holmgren and by Hanstrom, as will be shown in Chap. XVI, 
suggests that the protocerebrum and deutocerebrum are secondary 
subdivisions of a primitive preoral nerve mass evolved directly from the 


102 


PRINCIPLES OF INSECT MORPHOLOGY 


prostomial archicerebrum. According to this view the tritocerebral 
segment is the first true cephalic somite, and paired mesodermal cavities 
anterior to it are to be regarded as secondary in origin and not as repre- 
sentative of primitive metamerism in the antennal and preantennal 
regions of the head. As a corollary, the antennae and preantennae 
become prostomial appendages comparable with the cephalic tentacles 
of the annelid worms. The slightly postoral position of the antennal 
rudiments in the embryos of some insects (Fig. 23 B, C) has no morpho- 
logical significance, since the nerve centers of the appendages are not 
derived from the ventral postoral wall of the head. 

The accompanying diagram of the head segmentation (Fig. 64 A) 
expresses the idea that the tritocerebral segment is the first true somite of 
the arthropod trunk, and that the entire preoral region of the head bear- 
ing the labrum, the eyes, and the antennae is prostomial. The trito- 
cerebral ganglia are thus regarded as the first ganglia of the primitive 
ventral nerve cord (Fig. 244 B) ; they innervate the oral and labral regions 
and are connected with both the prostomial nerve mass and the first 
ganglion of the stomodaeal nervous system (FrGng) . 

The Gnathocephalon. — The gnathocephalon is the region of the insect 
head that supports the mandibles, the maxillae, and the labium. It con- 
tains, therefore, at least three somites (Fig. 54 A, 11, III, IV), and these 
somites are always distinct in the gnathal region of the embryo (Fig. 23 B, 
C). Some entomologists, however, have contended that lateral lobes of 
the hypopharynx, known as the superlinguae (Fig. 77 A, SUn), represent a 
pair of appendage rudiments behind the mandibles, and that the gnatho- 
cephalon, therefore, must contain four somites. Since the appendicular 
nature of the superlingue has not been demonstrated, only the three 
known segments of the gnathocephalon are recognized in the present 
discussion. The ganglia of the gnathal somites in insects are always 
combined in a single nerve mass, which is the ventral, or suboesophageal, 
ganglion of the definitive head. 

"ViTiile in the insects and the myriapods the gnathal segments are 
always completely united with the protocephalon in the adult head, there 
are many Crustacea, including the shrimps, crayfish, and crabs, in which 
the gnathal segments form a part of the body and are included with the 
segments of the maxilfipeds and pereiopods in a gnathothoracic section 
of the trunk, which may be covered by a carapace. In certain other 
crustaceans, however, as in the amphipods and isopods, the gnathal 
segments are united with the protocephalon in a composite head struc- 
ture resembling that of the insects. Intermediate stages occur in some of 
the phyllopods, as in Eubranchipus (Fig. 55 A), in which the mandibular 
segment {II) is intimately associated with the large protocephalon (Prc), 
while the maxillary segments {III -f IV), though united with each other. 



THE HEAD 


103 


form a distinct postcephalic region. The Arachnida cannot be said to 
have a head, since in most forms the protocephalon is combined vdth the 
folio-wing five segments to form the prosoma or anterior body section 
kno-wn as the cephalothorax. 

The Definitive Insect Head. — After the union of the gnathal segments 
with the protocephalon there are in general no visible marks in the result- 
ing head capsule showing the lines of fusion between the component 
segments. A subterminal groove (postoccipital suture) of the definitive 
cranium, however, surrounding the foramen magnum dorsally and 
laterally (Fig. 67 A, pos ) , is apparently the persisting suture between the 
first maxillary and the labial segments (Fig. 54 A, z) . In the adult head of 


(pos) 

I 
1 
I 


Pig. 54. — Diagrams illustrating the fundamental structure of the insect head. A, 
the composite definitive head formed by the union of at least four postoral somites (I, II. 
Ill, IV) with a preoral region derived from the cephalic lobes of the embryo (Fig. 23, Prc) 
including the prostomium and perhaps one or two (preantennal and antennal) somites 
(Fig. 244). B, relation of the head appendages to the head wall as shown in cross section, 

Machilis a transverse suture on the rear part of the head (Fig. 55 B, y), 
ending ventrally on each side between the bases of the mandible and the 
maxilla, may represent the suture between the mandibular and maxillary 
segments (Fig. 54 A, y), since it is suggestive of this suture in 
EubrancMpus (Fig. 65 A, y). Attempts have been made to determine 
the segmental limits in the cranial wall of insects by the muscle attach- 
ments, but the bases of the muscles appear in many instances to have 
migrated without regard to segment areas after the latter have become 
continuous. The various sutures that appear in the definitive head 
capsule, with the exception of the subterminal postoccipital suture, as 
shown by Smreczynski (1932) in Silpha obscura, have no relation to the 
original metamerism. 

Since the gnathal section of the definitive head is derived from the 
anterior part of the primitive segmented body of the insect, its surface 
regions must be homodynamous with those of the thoracic segments, 
and the parts of its appendages with those of the legs. The segment 







104 PRINCIPLES OF INSECT MORPHOLOGY 

structure of the head is best seen in a eross section (Fig. 54 B). The line 
on each side of the head through the articulations of the appendages with 
the lower edge of the cranium (a-a) clearly represents the dorso-pleural 
line of a more generalized part of the body (Fig. 35 C, o-o), and the lower 
ends of the appendage bases (6, 5) mark the pleuro-ventral line (6-5). 
The arch of the cranium, therefore, is the dorsum of the head (Fig. 54, D), 
an d its posterior sclerotization represents the fused terga of the gnathal 
segments. The true pleural areas of the head are the latero ventral 
membranous parts (B, P, P) in which are implanted the broad bases of 
the appendages (Cxpd). The venter is the lower wall of the head (F) 
between the appendage bases, including the hypopharynx (Hphy). The 
principal parts of the gnathal appendages are the limb bases {Cxpd), 


A 

Fig. 55. — Head of a primitive crustacean compared with that of an apterygote insect. 
A, Eubranchipus, showing gnathal segments (//, III, IV) distinct from the protocephalon 
(Prc). B, Mackilis, with procephalic and gnathal regions combined. 

which in the two pairs of maxillae bear each a pair of endite lobes {Bnd), 
and a segmented palpus, which is the telopodite {Tlpd). 

On the inner surfaces of the head walls there are developed various 
apodemal inflections of the cuticula, taking the form of ridges and arms, 
which together constitute the endoskeleton of the head. The most 
important endoskeletal feature consists of two pairs of apophyses that 
unite in pterygote insects to form the structure known as the tentorium, 
upon which arise the ventral muscles of the gnathal appendages and 
usually the muscles of the antennae. 

2. STRUCTURE OF THE DEFINITIVE INSECT HEAD 

The mature insect head, as we have seen, is a capsule in which all 
semblance of a segmented structure has been lost. Most of its sutures 
are secondary developments, being merely lines of cuticular inflections 
that form endoskeletal ridges. The dorsal, anterior, lateral, and posterior 
walls of the head are continuously sclerotized, forming a caplike cranium. 
The antennae in most adult insects occupy a lateral or anterior position 





TEE HEAD 


105 


on the cranial wall, but the gnathal appendages in the more generalized 
insects preserve their primitive lateroventral positions and articulate 
with the lower lateral margins of the cranium. 

According to the position of the mouth parts the head may assume 
one of three types of structure. If the gnathal appendages are directed 
downward, and the cranium corresponds in position to the body segments 
(Fig. 66 A), the head is said to be hypognathous. In many insects, how- 
ever, the cranium is turned upward on the neck so that the mouth parts 
are directed forward (B) and the head becomes prognathous. The 
hypognathous condition is the more primitive in the sense that it pre- 
serves the ventrolateral position of the appendages typical of ambulatory 
limbs, but it is possible, as claimed by Walker (1932), that the early 
insects were prognathous. The third, or opisthognathous, type results 
from a deflection of the facial region, giving the mouth parts a posterior 
ventral position, as in certain Homoptera. The three types of head 
structure relative to the position of the mouth are adaptations to different 
habitats or ways of feeding, and all may occur among closely related 
insects. 

General External Structure of the Head. — In an adult insect that 
preserves the hypognathous condition of the head, the facial area is 
directed forward (Fig. 56 B), the mandibles, maxillae, and labium project 
downward, and the labrum (Lm) hangs as a free lobe before the mouth 
from the ventral edge of the face. A pair of compound eyes {E) is 
located on the lateral or dorsolateral walls of the cranium, and usually 
three ocelli (A, 0) occur between them on the facial or dorsal area. Two 
of the ocelli are symmetrically placed laterad of the midline, the third is 
median and ventral or anterior to the others. The antennae {Ant) vary 
in their location from lateral points near the bases of the mandibles (Figs. 
55 B, 63 A, 64 A) to a more median site on the upper part of the face 
(Figs. 68, 63 C, D). The posterior surface of the head (Fig. 56 C) is 
occupied by the opening from the head into the neck, usually a large 
aperture (Figs. 59 B, 66 A), but sometimes much reduced by the encroach- 
ment of the posterior cranial walls (Fig. 65 B, C). The aperture is 
properly the foramen magnum by analogy with vertebrate anatomy, but 
in entomology it is commonly termed the “occipital” foramen. 

On the under surface of the head (Fig. 56 D) the areas occupied bji 
the bases of the mandibles and the maxfllae {MdC, MxC) take up th^ 
lateral regions; the labrum (Lm) projects from the anterior margin oi 
the cranium, and the base of the labium (Lb) lies transversely below tht 
foramen magnum. The true ventral area is that between the bases of 
the lateral appendages, the median part of which forms the variously 
modified lobe known as the hypopharynx {Hphy). Anterior to the 
hypopharynx and immediately behind the base of the labrum is the 



106 


PRINCIPLES OF INSECT MORPHOLOGY 


mouth (MtK). The posterior surface of the labrum sometimes bears a 
small median lobe called the epipharynx. It is clear that neither the 
epipharynx nor the hypopharynx has any relation to the pharynx, 
which is a part of the stomodaeal section of the alimentary canal. The 
two lobes are external head structures, but since their names have been 





Fig. 56. — Typical structure of the head of a pterygote insect, showing potential sutures 
and intersutural areas. A, anterior. B, lateral. C, posterior. D, ventral, appendages 
removed. a', a", a'", primary cranial articulations of mandible, maxilla, and labium; 
Ant, antenna; as, antennal suture; at, anterior tentorial pit; c, secondary anterior articu- 
lation of mandible; Clp, clypeus; cs, coronal suture; cvpl, cervical plates; Cvx, neck (cervix) ; 
E, compound eye; es, epistomal suture; For, foramen magnum; Fr, frons;/s, frontal suture; 
Ge, gena; Hphy, hypopharynx; HS, hypopharyngeal suspensorium ; Lb, labium; Lm, 
labrum; Md, mandible; MdC, mandibular cavity; Mx, maxilla; MxC, maxillary cavity; 
O, ocelli; Oc, occiput; occ, occipital condyle; ocs, occipital suture; os, ocular suture; Pge, 
postgena; Poc, postocciput; pos, postoccipital suture; SIO, orifice of salivary duct; sgs, 
subgenal suture; Vx, vertex. 


handed down from earlier days in entomology they have become a part 
of our accepted vocabulary. The space enclosed by the labrum and the 
mouth appendages is sometimes called the "mouth cavity,” but inasmuch 
as it lies entirely outside the body it is merely an intergnathal space and 
should be termed the preoral, or extraoral, cavity (Figs. 60, 155, PrC). 

Sutures of the Cranium. — The endoskeletal ridges of the head are 
marked on the surface of the cranium by grooves which are known as the 



THE HEAD 


107 


head "sutures/’ One of these grooves, the postoccipital suture, lying 
close to the posterior margin of the head (Fig. 56 B, C, pos), probably 
marks the intersegmental line between the primitive first and second 
maxillary segments. The other head sutures commonly present in 
pterygote insects are all apparently secondary inflections of the cuticula. 
Some of them form apodemal ridges of functional importance but having 
no segmental significance. 

The Epicranial Suture . — ^This suture is a distinctive feature of the 
insect head, though it is not fully developed in Apterygota and is more 
or less suppressed in many Pterygota. Typically it has the form of an 
inverted Y with the stem placed medially on the top of the head, and the 
arms diverging downward on the face. The dorsal part is known as the 



Fig. 57. — Diagrams illustrating the principal sutures and areas of the insect craniuml 
and two potential sutures in the frontal region. AntS, antennal socket; ASc, antenna, 
sclerite; hs, hypostomal suture (poatmandibular part of subgenal suture); Hst, hypostoma; 
Oc, occipital arch (occiput and postgena); O&c, ocular sclerite; p/s, postfrontal suture; 
Plst, pleurostoma; Prtl, parietal; ps, pleurostomal suture (supramandibular part of sub- 
genal suture). (Fig. 56.) 

coronal^ or metopic, suture (Figs. 56 A, 57 B, cs). The facial arms branch- 
ing from the coronal suture are apparently not homologous in all cases. 
Those more usually present diverge above the median ocellus and proceed 
ventrally on the face mesad of the antennal bases toward the anterior 
articulations of the mandibles. These sutures may be defined as the true 
frontal sutures (/s). In most insects, however, the frontal sutures are 
incomplete (Figs. 58 A, 59 A), and often they are entirely absent. In 
certain orthopteroid insects two sutures diverge from the coronal suture 
above the lateral ocelli and extend a varying distance laterally and ven- 
trally on the face laterad of the antennal bases. These sutures are the 
postfrontal sutures (Fig. 57 B, pfs). They are particularly prominent in 
some Dermaptera but are present in Plecoptera, especially in larval stages, 
and they are weakly developed in Phasmidae and Mantidae. In some of 
these insects there is also a suggestion of the frontal sutures (fs). Since 
generally, however, the two pairs of sutures do not occur in the same 



108 


PRINCIPLES OF INSECT MORPHOLOGY 


species, the postfrontal sutures have often been confused with the frontal 
sutures, but their independence has been clearly shown by Crampton 
(1932). 

The Occipital Suture. — A suture of the head developed particularly 
in orthopteroid insects is the occipital suture (Figs. 56 B, C, 57 A, ocs). 
This suture crosses the back of the head and suggests the posterior head 
suture of Machilis (Fig. 55 B, y), but it ends ventrally on each side of the 
epicranium before the posterior articulations of the mandibles (a'). The 
occipital suture forms internally a ridge which serves probably to 
strengthen the posterior parts of the epicranial walls. 

The Postoccipital Suture. — This suture lies on the extreme posterior 
part of the cranium where it closely surrounds the foramen magnum 
dorsally and laterally (Figs. 56 B, C, 57 A, pos). Internally the post- 
occipital suture forms a strong postoccipital ridge (Figs. 58 B, 59 B, PoR), 
often produced into apodemal plates, upon which are attached the 
anterior ends of the prothoracic and neck muscles that move the head. 
The posterior arms of the tentorium arise from the ventral ends of the 
postoccipital ridge, and the points of their invagination appear externally 
as pits in the lower ends of the postoccipital suture (Figs. 56 B, C, 

57 A, 58 B, 59 B, pt). 

The Subgenal Sutures. — On each side of the head close to the lower 
edge of the lateral cranial wall is a subgenal suture (Figs. 56 A, B, 57 A, 

58 B, sgs). It usually follows the contour of the cranial margin, but in 
some insects it is arched upward over the mandible (Fig. 58 A). On 
the inner surface of the cranium this suture forms a submarginal subgenal 
ridge (Figs. 54 B, 58 B, SgR), which usually extends from the posterior 
tentorial pit (Fig. 56 B, pt) to a point just above the anterior articulation 
of the mandible (c). The two subgenal ridges strengthen the cranial 
walls along the lines of attachment of the gnathal appendages. For 
descriptive purposes the part of the subgenal suture lying above the 
mandible is sometimes distinguished as the pleurostomal suture (Fig. 57 A, 
ps), and the part posterior to the mandible as the hypostomal suture (hs). 

The Epistomal Suture. — In many pterygote insects the anterior ends 
of the subgenal sutures are connected across the lower part of the face 
by an epistomal suture (Figs. 56 A, B, 57, es). This suture is often 
a deep inflection producing internally a strong epistomal ridge (Figs. 58 B, 
76 A, B, C, ER), which typically forms a brace in the region between the 
anterior mandibular articulations. As we shall later see, however, the 
epistomal suture and its ridge are subject to much variation in position. 
In pterygote insects the anterior arms of the tentorium usually arise from 
the epistomal ridge, and their external pits (at), lying in the epistomal 
suture, serve to identify this suture when the latter is likely to be confused 
with neighboring sutures. 



THE HEAD 


109 


The Ocular Sutures. — These sutures are grooves which frequently 
surround the compound eyes (Figs. 56 A, 57, os), each forming internally 
a skeletal ridge around the edge of the retina. 

The Antennal Sxitures. — Each antenna is set in a membranous area of 
the cranial wall, known as the antennal socket, the rim of which is 
reinforced by a submarginal ridge. The external groove of this ridge is 
the so-called antennal suture (Figs. 56 A, 57, as). 

Other Head Sutures. — In addition to the sutures described above, 
other grooves of less constant occurrence may occur on the head wall. 
In the cockroach, for example (Fig. 58 A), a prominent subantennal 
suture (sas) extends downward from each antennal suture to the subgenal 



Flo. 58. — External and internal structures of the insect cranium. A, head o{ Blalta 
oricnlalis (note absence of epistomal suture, B, cs) between anterior tentorial pits. B, 
diagram of relation of endoskelotal structures of head to the cranial sutures. A.T^ at^ 
anterior tentorial arm and pit; DT^ dorsal tentorial arm; ER^ es, epistomal ridge and 
suture; Poi2, pos, postoccipital ridge and suture; SgR^ sgs, subgenal ridge and suture; TB^ 
Vtj tentorial bridge and posterior tentorial pit. 

suture. It is sometimes called the “frontogenal suture,” but its lateral 
position makes it doubtful if it defines the lateral limits of the region of 
the true frons (Fr). In the cricket (Fig. 59 A) a subocular suture (sos) 
extends from the lower angle of each compound eye to the subgenal 
suture above the anterior mandibular articulation. In the higher 
Diptera a ptilinal suture cuts across the region of the frons just above the 
bases of the antennae, which marks the groove where the pupal ptilinum 
was permanently withdrawn. Still other grooves frequently appear 
in the cranial walls but they are too variable to be included in a general 
account of the head sutures. 

The Tentorial Pits. — The four points on the cranial wall where the 
arms of the tentorium are invaginated are never entirely closed, and the 
external depressions at the tentorial roots are such important landmarks 
in the head structure that they deserve special consideration. The 
depressions are conveniently called “pits” since they are usually pitlike 



no 


PRINCIPLES OF INSECT MORPHOLOGY 


in form, but frequently they are elongate slits, and sometimes suture-like 
grooves. 

The Anterior Tentorial Pits. — The anterior arms of the tentorium 
in pterygote insects always arise from some part of the subgenal or 
epistomal ridges, and their roots appear externally as depressions in the 
corresponding sutures. In the more generalized pterygote insects the 
anterior tentorial pits or fossae lie in the subgenal sutures above the bases 
of the mandibles or above the anterior mandibular articulation (Figs. 
58, 59 A, at); but in the majority of cases they are contained in the 
epistomal suture (Figs. 56 A, 57, at), though their position in the latter is 
subject to much variation. A facial suture that contains the tentorial 
pits, therefore, is usually to be identified as the epistomal suture. If the 
bases of the anterior tentorial arms are broad, the pits are correspondingly 
elongate, as in the cricket, where each anterior “pit” is a long slit (Fig. 
59 A, at) contained in both the subgenal and epistomal sutures. 



The Posterior Tentorial Pits. — The posterior arms of the tentorium 
arise from the ventral ends of the postoccipital ridge, and their pits 
lie in the lower extremities of the postoccipital suture (Figs. 56 B, C, 
57 A, 59 B, 'pt). If the head is of the prognathous type, the posterior 
pits are usually drawn forward on the ventral side of the head (Fig. 66 B, 
pt), and the lower ends of the postoccipital suture {pos) are correspond- 
ingly lengthened behind them. The posterior pits also may take the 
form of slits or grooves. In the soldier termites the sutural lines at the 
sides of the elongate base of the labium (Fig. 69 A, pt, pt) are the roots of 
the extended posterior arms of the tentorium (C, PT). 

The dorsal arms of the tentorium, when present, are often united with 
the epicranial wall in the neighborhood of the antennae, and the points of 
union are sometimes marked by depressions or dark spots (Fig, 63 B, C, 
dt), which may be distinguished as tentorial maculae, since they are usually 
not true invagination pits. 


THE HEAD 


111 


The Areas of the Cranium. — The cranial areas set off by the head 
sutures are quite definite features of the head structure when the sutures 
are complete, and are given distinctive names, but their limits become 
obscure when the sutures are obsolete or suppressed. The cranial areas, 
in themselves, probably have little or no significance; their demarkation 
is incidental to the presence of the sutures, which in turn are but the 
external grooves of endoskcletal ridges, which are the important structural 
features. The intersutural areas, however, sometimes called the head 
“sclerites,” serve as convenient characters for descriptive purposes. The 
principal areas of the cranium arc the median facial fronioclypeal area, the 
lateral parietah, the occipital arch, the posloccipiit, and the narrow suh~ 
genal areas above the bases of the gnathal appendages. 

The Fronloclypeal Area. — This area of the head is typically the facial 
region between the antennae, or between the frontal sutures when the 
latter are present, and the base of the labrum. When the epistomal 
suture is present the frontoclypeal area is divided into a dorsal or posterior 
from (Figs. 56 A, 57, Fr), and a ventral or anterior clypeiis (Clp). 
The frons bears the median ocellus on its upper part and is the area of the 
head on which the muscles of the labrum, when present, with rare excep- 
tions (Diptera) , take their origin. '^Hien the frontal sutures are complete, 
as in many coleopterous larvae, the frons is a well-defined sclerite. 
The antennae are never located on the true frontal area, but their 
bases may become approximated medially and constrict the frons between 
them. In some insects the frons is greatly reduced by an upward 
extension of the clypeus. The clypeus is the region of the cranial wall on 
which the dorsal dilator muscles of the extraoral “mouth cavity” 
(cibarium) take their origin, and in general it varies in size according to 
the size of these muscles, being greatly enlarged in some of the sucking 
insects (Fig. 177), but in the caterpillars also it becomes extended dorsally 
as a prominent triangular plate (Fig. 64 A, Clp). The clypeus is often 
subdivided into a postclypeus and an anteclypeus, even though the 
postclypeal part, as in the cockroach (Fig. 58 A), may not be separated 
from the frons. 

The Parietals. — The lateral areas of the cranium, separated above 
by the coronal suture, are the parietals (Fig. 57 A, Prtl). They are 
bounded anteriorly and posteriorly by the frontal and occipital sutures, 
respectively, when these sutures are present. Each parietal area beai's 
an antenna, one of the lateral ocelli, and a compound eye. The dorsal 
surfaces of the two parietals forming the top of the head constitute the 
vertex (Fig. 56 A, B, Vx). In some insects there is a prominent angle, 
the fastigium, between the vertex and the face. The lateral parts of the 
parietals beneath or behind the eyes are the genae (Ge). The narrow 
bands encircling the compound eyes within the ocular sutures (os) are 



112 


PRINCIPLES OF INSECT MORPHOLOGY 


known as the ocular sclerites {OSc), and the marginal areas of the antennal 
sockets, defined by the antennal sutures (as), are termed the antennal 
sclerites {A Sc). 

The Occipital Arch. — On the posterior surface of the head (Fig. 56 C) 
the horseshoe-shaped band between the occipital and the postoccipital 
sutures {pcs, pos) is the occipital arch {Oc, Pge). Generally the term 
occiput (Oc) is given to the dorsal part of this area, and the lateral parts, 
l 3 dng posterior to the genae, are called the postgenae (B, C, Pge). Rarely 
the occiput and the postgenae are separated by a suture on each side 
of the foramen magnum. Since the occipital suture is frequently 
imperfect or absent, the occipital and postgenal areas can generally 
be defined only as the posterior region of the cranium. 

The Postocciput. — This is the narrow posterior rim of the epicranium 
(Figs. 56 C, 57 A, Poc) set off from the occipital arch by the postoccipital 
suture (pos), and to which the neck membrane is attached. Ventro- 
laterally the posterior margin of the postocciput may be produced on 
each side in a small process, the occipital condyle {occ) , to which is articu- 
lated the anterior cervical sclerite (Fig. 56 B, cvpl). The postocciput 
probably is a sclerotic remnant of the labial segment. It is often so 
narrow as to be scarcely perceptible, and sometimes it is nothing more 
than the posterior lip of the inflection forming the postoccipital ridge. 

The Subgenal Areas. — The narrow marginal areas on the sides of 
the cranium below the subgenal sutures (Fig. 57 A), on which the gnathal 
appendages are articulated, are important though often inconspicuous 
features of the cranium, and their modifications have a distinctive 
character in many groups of insects. The part of each subgenal area 
above the mandible is distinguished as the pleurostoma (Plst), and the 
part behind the mandible as the hypostoma {Hst). The hypostomal 
areas, set off by the hypostomal sutures {hs), are sometimes extended 
mesally on the ventral side of the head, forming, as in the caterpillars 
(Fig. 164 C, Hst), a pair of hypostomal lobes of the cranium cutting into 
the base of the labium. In other cases, as in adult Hymenoptera (Fig. 
65, B, C), Diptera, and Heteroptera, they may be approximated or 
completely united in a hypostomal bridge closing the ventral end of the 
foramen magnum behind or above the base of the labium. The pleuro- 
stoma is sometimes enlarged by an arching of the pleurostomal suture 
above the base of the mandible. When the subgenal sutures are con- 
nected across the face by an epistomal sutiue (Figs. 56 A, 57, es), there 
is set off a ventral marginal rim of the epicranium known as the peristome. 
In some of the higher Diptera the peristome surrounds a ventral depres- 
sion of the head into wliich the proboscis is retractile. The cl 3 q}eus, or its 
upper part, considered as a part of the peristome, is sometimes called the 
epistoma, though this term is applied also to the epistomal ridge, especially 



THE HEAD 


113 


when the latter appears as a conspicuous dark band on an otherwise 
weakly sclerotized cranium (Fig. 161 E, H, Est). 

The Labrum. — ^The labrum of generalized insects is usually a broad 
fiat lobe of the head (Fig. 59 A, Lm) that hangs downward before the 
mandibles and forms the anterior wall of the intergnathal preoral cavity. 
Its external surface is attached proximally to the lower margin of the 
clypeus by the clypeolabral suture, wiiich gives mobility to the labrum. 
Its inner surface is continuous with the membranous inner surface 
of the clypeal region of the head (Fig. 60 A), the two forming the so-called 
epipharyngeal wall of the preoral cavity, which ends proximally at the 
mouth aperture (Mth). A median lobe sometimes developed on the 
epipharyngeal wall may form a more or less distinct epipharynx (Fig. 
163 B, Ephy). In the lateral angles between the labrum and the clypeus 
there is generally present a pair of small sclerites, the tormae, which 
usually extend into the epipharyngeal surface of the clypeus. The form 
of the labrum varies much in insects with specialized mouth parts, but 
the organ seldom loses its essential structure of a movable preoral lobe 
of the head. 

The musculature of the labrum includes muscles situated within the 
labrum itself, and cranial muscles inserted on its base. Within the 
labrum is a single or paired median muscle, the compressor of the labrum 
(Fig. 60 A, cplr), attached on the anterior and posterior walls. The 
labrum is movable usually by two pairs of long muscles taking their 
origins on the upper part of the frons. One pair, the anterior labral 
muscles {mlra), is inserted on the anterior margin of the labral base, 
sometimes laterally, sometimes medially; the other pair, the posterior 
labral muscles (mlrp), is inserted posteriorly, usually on the epipharyngeal 
processes of the tormae. The labrum, therefore, is variously movable, its 
potential motions being those of retraction, production and reduction, 
and of lateral movements. Either pair of labral muscles may be absent, 
and in some cases both pairs are absent; in adult Diptera the labral 
muscles are exceptional in that they take their origins on the clypeus. 

The Hypopharynx. — The ventral wall of the gnathal region of the 
head in the more generalized insects is mostly occupied by the large 
median lobe known as the hypopharynx (Figs. 54 B, 56 D, Hphy). The 
mouth (Fig. 56 D, Mth) lies between the base of the clypeus and the 
hypopharynx, and usually the opening of the salivary duct {SIO) is 
situated in a pocket between the hypopharynx and the base of the labium 
(Lb), but in some insects the salivary orifice is placed on the hypopharynx, 
as in adult Hemiptera and Diptera, in which the hypopharynx is traversed 
by a salivary canal. In many insects, especially the more generalized 
forms, the hypopharynx includes a pair of lateral lobes known as the 
superlinguae (Fig. 77 C, D, Blin), w'hich have united with the median 



114 


PRINCIPLES OF INSECT MORPHOLOGY 


part of the hypopharynx, or lingua {Lin), to form the definitive organ. 
The superlinguae in some of the Apterygota arise in the embryo separate 
from the median lingua (A) in a position close to the bases of the man- 
dibles {Md). 

The usual form of the hypopharynx in orthopteroid insects is that 
of a thick, sometimes irregular lobe (Fig. 60 A, Hphy), lying like a tongue 
in the preoral cavity, where it is attached to the head between the mouth 
and the labium. Its posterior wall is reflected into the adoral wall of 



Fig. 60. — The preoral cavity and the hypopharynx. A, section through region of 
labrum, hypopharynx, mouth, and salivary orifice of Gryllus, showing the food pocket 
(cibarium, Cb) and the salivary pocket (salivarium, Slv) of the preoral cavity and their 
related muscles. B, base of hypopharynx, cibarium, and mouth of Dissosteira. 

the labium usually at the base of the prementum, and there is formed 
here a small salivary pocket, or salivarium (Figs. 60 A, 155, Slv), into 
which opens the duct of the labial glands {SID). The lateral walls of 
the hypophar 3 mx contain a pair of basal plates or bars {w), the posterior 
ends of which may extend to the salivarium. On these sclerites are 
inserted the retractor muscles of the hypopharynx {rhphy), which take 
their origins on the tentorium. The adoral surface of the hypopharynx 
is differentiated into a distal sclerotized area and a proximal more 
membranous part. The latter presents a median depression, the floor 
of which is directly continuous through the mouth with the ventral wall 
of the stomodaeum and itself forms the floor of a special preoral food 
chamber, the cibarium {Cb), beneath the opposing epipharyngeal wall 
of the cl 5 T)eus. 

The adoral area of the h 3 rpopharynx is flanked on each side by a 
sclerite (Fig. 60 B, HS) or a group of sclerites (A, HS), the pair or paired 


THE HEAD 


115 


groups of which constitute the suspensoria of the hypopharynx (fuUurae, 
Zungensiabchen). Each suspensorium in its simpler form of a single 
sclerite (B, HS) articulates distally with the anterior end of the lateral 
sclerite {w) of the hypopharynx, and the proximal end (y) enters the 
lateral angle of the mouth where it ends in the stomodaeal wall and gives 
insertion to the retractor muscle of the mouth angle (rao), which takes 
its origin on the frons (A). The suspensorial sclerites are subject to 
much variation in form in different insects, and they may be entirely 
absent. In Dissosteira (B) each suspensorial rod has a proximal branch 
{x) that extends laterally to the base of the adductor apodeme of theman- 



Fio, 61. — Examples of the presence of ventral hypopharyngeal apodemes {HA) in a 
centipede and an apterygote insect, ■which evidently represent the anterior arms of the 
usual insect tentorium. A, under surface of head of Lithobius, mouth appendages 
removed, showing hypopharyngeal suspensorium (HS) and apodemes (HA). B, hypo- 
pharynx and right maxilla of Heierojapyx, ventral view, showing long hypopharyngeal 
apodemes on which are attached muscles of maxillae and labium. 


dible, and a similar mandibular branch is present in Blatta, but in the 
cricket (A) the suspensorial apparatus consists of a group of sclerites 
(HS) on each side. 

It is possible that the suspensorial area of the hypopharynx should 
not be regarded as a part of the true hypopharynx. Morphologically 
it lies between the mouth and the bases of the mandibles, and hence it 
may represent the venter of the postoral tritocerebral somite of the 
head. In the acridid Dissosteira (Fig. 60 B) there project from its walls 
between the forks of the suspensorial rods (HS) of the hypopharynx, 
anterior to the mandibles, a pair of small lobes (1), which are suggestive 
of being remnants of the postantennal appendages. 

In the Chilopoda the h 3 rpopharynx is a small postoral lobe of the 
ventral head wall (Fig. 61 A, Hphy), but it is supported by a pair of large 
suspensorial plates (HS) that extend laterally and are connected with the 



116 


PRINCIPLES OF INSECT MORPHOLOGY 


ventral edges of the cranium at points {d, d) before the bases of the 
mandibles. From each plate there is given off posteriorly an apodemal 
process (HA) into the head cavity at the side of the stomodaeum. It is 
possible, as we shall see presently, that these hypopharyngeal apophyses 
of the chilopods are related to the anterior tentorial arms of insects. 

The Tentorium. — In pterygote insects the lower edges of the epicranial 
walls are braced by an endoskeletal structure known as the tentorium. 
The tentorium is formed by two pairs of cuticular invaginations that 
unite within the head to compose a framework arching over the ventral 
nerve cord, but passing beneath the stomodaeum and supporting the 
latter. The component invaginations are the anterior tentorial arms and 
the posterior tentorial arms. The first arise from the anterior tentorial 
pits in the subgenal sutures or in the epistomal suture; the second take 
their origins from the posterior tentorial pits in the lower ends of the 
postoccipital suture. Very commonly the tentorium includes also a 
pair of dorsal arms (Figs. 58 B, 62 B, DT) extending from the anterior 
arms to the head wall near the bases of the antennae. The dorsal arms 
appear to be secondary outgrowths of the anterior arms, since, though 
sometimes firmly united with the cranial cuticula, they are often attached 
only to the epidermis, and their origin as external ingrowths has not been 
demonstrated. 

In orthopteroid insects the tentorial arms unite in the middle of the 
back part of the head cavity to form an X-shaped structure (Fig. 76 B), 
the central part of which may become expanded, and in such cases the 
shape of the tentorium suggests a tent, as the name implies, or a canopy 
braced by four stays. In many other insects, however, the tentorial 
structure has the form of the Greek letter tt, inverted when seen from 
behind (Fig. 62 A), the posterior arms being continuous in a transverse 
bar, or tentorial bridge (TB), through the back of the head, with the 
anterior arms (AT) attached to it near its outer ends. A study of 
the apparent origin of the tentorium in the Apterygota suggests that the 
TT-form, rather than the X-form, is the more primitive type of tentorial 
structure (Fig. 58 B). 

The shape of the pterygote tentorium is subject to numerous modifica- 
tions. The central part of the structure may be enlarged to form a 
broad plate, the so-called “corporotentorium” (Fig. 62 C, CT). In 
some of the Orthoptera a pair of median processes arises at the bases of 
the anterior arms (C, o); in Blattidae these processes are united in an 
anterior bridge before the circumoesophageal connectives, which thus 
pass through an aperture in the central plate of the tentorium. The 
bridge composed of the posterior arms attains an excessive development 
in the termite soldier (Fig. 69 B, C), where it forms a long rooflike 
structure (FT) over the basal plate of the labium (Pmt), enclosing a 



THE HEAD 


117 


triangular channel traversed by the ventral nerve connectives and the 
salivary ducts extending from the thorax into the head; from its anterior 
end the narrow anterior arms (AT) diverge to the facial wall of the head in 
the usual manner. The relative size of the anterior arms and the bridge 
varies much in different insects. In some the anterior arms form strong 
braces expanded against the subgenal or epistomal sutures, and in such 
cases the bridge may be reduced to a nari-ow bar (Fig. 62 D, TB) between 
the posterior tentorial pits; but, again, the anterior arms are slender 
rods or are reduced to mere threads (E, F), especially when the posterior 
bridge is large, and not unfrequently in such cases they appear to be 
entirely suppressed or represented only by a pair’ of spurs on the anterior 
margin of the bridge. Finally, the bridge itself may be incomplete and 
the posterior arms reduced to a pair of small processes (F, PT), either in 





Fig. 62. — Various modifications of the tentorium. AT, anterior tentorial arms; CT, 
corporotentorium; DT, dorsal arms; PT, posterior arms; TB, tentorial bridge. 


conjunction with well-developed anterior arms or entirely isolated by the 
suppression of the latter. In rare cases there is no trace of a tentorial 
structure. 

Besides bracing the cranial walls, the tentorium gives attachment to 
the ventral adductor muscles of the mandibles, maxillae, and labium, 
to the retractors of the hypopharynx, and to the ventral dilators of the 
stomodaeum. All these muscles are muscles that, it would seem, should 
take their origins on the sterna of the gnathal segments or on apodemal 
processes of the sterna, and yet the tentorium appears to be a tergal 
structure, since its arms in pterygote insects arise from the cranial walls 
dorsal to the bases of the gnathal appendages. In most insects the anten- 
nal muscles also arise on the tentorium, particularly on the dorsal arms. 

A study of the Apterygota and Chilopoda suggests an explanation of 
the seeming anomaly in the relation of the pterygote tentorium to the 
ventral head muscles. In the Japygidae (Fig. 61 B) the ventral muscles 



118 


PRINCIPLES OF INSECT MORPHOLOGY 


of the gnathal appendages arise upon two long endoskeletal arms {HA) 
that spring from the base of the hypopharynx, where they are supported 
by two short divergent bars {HS). A similar structure, as we have seen, 
exists in the Chilopoda (A), but here the suspensoria of the apodemal 
arms {HA) are long sclerites {HS) that extend laterally to the ventral 
edges of the cranium, to which they are attached at points {d, d) anterior 
to the bases of the mandibles. In some of the Apterygota and in the 
Chilopoda, therefore, the ventral muscles of the gnathal appendages 
arise from a pair of endoskeletal processes that are clearly sternal apophy- 
ses of the gnathal region of the head. We can scarcely avoid the con- 
clusion, then, that these sternal apophyses are in some way homologous 
with the anterior arms of the pterygote tentorium. 

Sufficient evidence is not yet at hand to show the exact manner by 
which the hypopharyngeal sternal apophyses, which retain their primitive 
ventral position in some of the Apterygota, may have been transposed 
in the Pterygota to the facial region of the cranium, where evidently 
they have become the anterior arms of the tentorium. But the fact 
that in many of the lower Pterygota the anterior tentorial arms take their 
origins laterally in the subgenal sutures, and not in the epistomal suture, 
suggests that the apophyses have migrated first to this position, and 
subsequently to that in the epistomal suture. In the larvae of Ephemer- 
ida the anterior tentorial arms arise from deep lateral grooves of the 
head wall just above the bases of the mandibles; these grooves evidently 
represent the subgenal sutures of higher insects; but in Lepisma the 
anterior tentorial roots lie in the membranous areas between the bases of 
the mandibles and the lateral margins of the clypeus. 

The pterygote tentorium, it thus appears, is a composite structure 
formed of tergal and sternal elements. Its anterior arms, bearing the 
attachments of the ventral muscles of the head appendages, are primi- 
tively sternal apophyses, analogous to the sternal apophyses of the thorax, 
that have secondarily migrated to the lateral or facial walls of the head; 
the posterior bridge is formed by a pair of tergal apodemes arising in the 
suture between the maxillary and labial segments. After the union of 
the anterior arms with the posterior bridge, the muscles of the former have 
migrated to all parts of the tentorium, and even in some cases to the 
adjoining cranial walls. The muscles of the antennae evidently have 
gained access to the tentorium by way of the secondary dorsal arms. In 
the more generalized condition found in some insect larvae the antennal 
muscles arise on the dorsal walls of the cranium. 

3. SPECIAL MODIFICATIONS IN THE STRUCTURE OF THE HEAD 

The important structural modifications of the head affect principally 
the frontoclypeal area and the posterior lateral and ventral regions. 



THE HEAD 


119 


Modifications in the facial plates are often correlated with variations in 
the relative size of the buccal and pharyngeal parts of the stomodaeum 
or with the special development of the cibarium as an organ of sucking. 
Modifications in the posterior ventral parts of the head are associated with 
a flattening and elongation of the cranial capsule, usually resulting from 
an upward revolution of the head on the neck by which the mouth parts 
become directed forward, and, in certain orders, are accompanied by an 
elongation of the base of the labium or the addition of a gula to its proxi- 
mal part. 

Modifications in the Frontoclypeal Region of the Head. — The frons 
and the clypeus, as we have seen, are not always clearly defined or 
delimited cranial areas, since the epistomal suture is frequently absent, 
as in the head of the cockroach (Fig. 58 A), and the frontal sutures them- 
selves are generally obsolete or suppressed. Moreover, even when the 
facial sutures are present they may depart so vudely from their typical 
positions that they cannot readily be identified. In such cases the stu- 
dent must consult other characters than the sutures for determining the 
true frontal and clypeal areas. 

When the epistomal suture is absent, the anterior tentorial pits lie in 
the anterior ends of the subgenal sutures (Fig. 58, sgs); but when the 
epistomal suture is present, the pits are usually in tliis suture (Fig. 63 A, 
at, at) regardless of its displacement. The epistomal suture, when 
present, therefore, is to be identified as the suture of the face containing 
the anterior tentorial pits. Additional evidence of the identity of the 
clypeal and frontal areas, especially valuable when the epistomal suture 
is absent, may be deduced from the attachments of certain muscles on 
the cranium. The clypeus is the area upon which arise the dilator 
muscles of the cibarium and the dorsal muscles of the buccal cavity, or 
that part of the stomodaeum just witliin the mouth (Fig. 60 A, dlcb, 
dlbc) . These muscles lie anterior to the frontal ganglion (Fig. 155, FrGng) . 
The frons gives attachment to the muscles of the labrum (Fig. 60, mlra, 
mlrp) and to stomodaeal dilators lying posterior to the frontal ganglion 
(Fig. 155). The frons also bears, usually in its upper angle, the median 
ocellus (Fig. 56 A) when the latter is present. By following these prin- 
ciples a consistent identification of the facial cranial areas becomes pos- 
sible in most cases. 

As long as the epistomal suture maintains its direct course across the 
face (Fig. 63 A), no complications arise; but the suture is frequently 
arched upward, and this change in the position of the suture extends the 
clypeus into the facial region and reduces the area of the frons. A modi- 
fication of this kind has taken place in the Hymenoptera, as seen in the 
larval head of Vespa (B), in the adult of the currant sawfly (C), and in 
the honey bee (Dl , where the clypeus extends upward almost to the bases 



120 


PRINCIPLES OF INSECT MORPHOLOGY 


of the antennae. The arched epistomal suture (es) is to be identified by 
the tentorial pits {at, at) in its course. In the caterpillars the clypeus 
takes on a triangular form (Fig. 64 A) and has generally been mistaken 
for the frons. That the triangular plate of the caterpillar’s face is the 
clypeus, however, is attested by the facts that the anterior tentorial 
arms arise from its lateral sutures (B, AT) and that the dorsal dilator 
muscles of the mouth cavity have their origins on its inner surface. The 
labral muscles {mlrp), on the other hand, arise as usual on the true 
frontal area, which in the caterpillar is invaginated above the apex of the 



Fig. 63. — ^Various types of heads showing modification and suppression of the irontal 
sutures. A, larva of Popillia japonica. B, larva of Vespa, C, adult of Pteronidea 
ribesii. D, adult of Apis mellifica. 


clypeus (Fr). The clypeus attains its greatest development in some of 
the Hemiptera (Fig. 177), where it often forms a large bulging plate of the 
face, which accommodates the huge dilator muscles of the sucking pump. 

The frons is typically a triangular area narrowed dorsally between 
the converging frontal sutures (Fig. 63 A, Fr), and it usually preserves its 
triangular shape, but its limits are generally obscured by a partial or 
complete suppression of the frontal sutures. The true frontal region, 
however, is to be identified by the location of the median ocellus and the 
origins of the labral muscles upon it. It always lies between the antennae, 
though it may be greatly narrowed or constricted by the approximation 
of the bases of these appendages. In the larval head of Vespa (B) the 
frons is distinct (Fr), but it is reduced in size by the upward extension of 
the cl 3 ^eus. In the adult head of Pteronidea (C) the frontal sutures are 



THE HEAD 


121 


obsolete ventrally where the frontal region is narrowed between the 
approximated bases of the antennae. In the adult honey bee (D) the 
frontal sutures are obliterated, but the frontal region (Fr) must extend 
upward from the clypeus to include the median ocellus, and upon this 
region, just above the bases of the antennae, are attached the muscles of 
the labrum. In the Hemiptera the Irons may be apparent as a small 
triangular area upon which is located the median ocellus (Fig. 177 B, Fr), 
but usually it is not defined from the vertex, though the latter may be 
greatly enlarged (F, H) or greatly reduced (J). 

The Irons suffers an extreme modification in the caterpillar head. 
It is here transformed into the shape of an inverted Y by the upward 
growth of the clypeus into its ventral part. The stem of the Y, moreover, 
has sunken into the head by a median invagination of the head wall 



Fig. 64. — Head of a caterpillar. A, Lycophotia margaritosa, anterior view. B, 
Prionoxystus robiniae, interior of the head from behind, showing invaginated frons (Fr) on 
which the labral muscles (mlrp) are attached. 

above the apex of the clypeus. The dorsal part of the frons, therefore, 
is to be seen internally as a deep fold or ridge (Fig. 64 B, Fr) upon which 
arise the labral muscles (mlrp). The ventral parts, or arms of the 
frontal Y, are apparently the narrow strips between the clypeus and the 
parietals, which entomologists usually term the “adfrontals” (A, fr). 
The true vertex of the caterpillar’s head is cut out by the posterior 
emargination of the dorsal wall of the cranium. 

Modifications in the Posterior Ventral Region of the Head. — In the 
more generalized type of cranial structure, as in the Orthoptera (Fig. 
59 B), there is no ventral sclerotization of the head wall between the 
foramen magnum and the base of the labium, the submentum (Smt) 
being direcrly continuous with the neck membrane between its lateral 
attachments to the cranial margins just behind the posterior tentorial 
pits. In many insects, however, the postgenal regions of the cranium 
are greatly lengthened anteroposteriorly, and a long space intervenes 
between the foramen magnum and the mandibles. Two different types 
of structure are developed upon this modification, both of which 



122 


PRINCIPLES OF INSECT MORPHOLOGY 


involve a sclerotic separation of the true base of the labium from the 
foramen magnum. In one type the separation is formed by the mesal 
extension of hypostomal lobes of the cranial walls forming a more or less 
complete ventral hypostomal bridge proximal to the labium; in the other 
a sclerotization of the gular region of the neck, uniting the ventral 
margins of the cranium proximal to the posterior tentorial pits, is added 
to the base of the labium and constitutes a ventral plate of the head wall 
known as the gula. 

The Hypostomal Bridge . — Mesal extensions of the hypostomal areas 
of the cranium are a characteristic feature of the head of lepidopterous 
larvae; in adult Hymenoptera, Diptera, Heteroptera, and certain other 
insects the hypostomal lobes are united in a complete bridge closing the 
foramen magnum ventrally. 



Fig. 65. — Heads of Hymenoptera, showing evolution of the hypostomal bridge formed 
by united median lobes of the postgenal regions. A, larva of Vespa. B, adult of 
Vespa. C, adult of Apis mellifica. 

On the ventral side of a caterpillar’s head (Fig. 164 C) it will be seen 
that the base of the labiomaxillary complex is separated from the neck 
by two sclerotic lobes (Hst) extending mesally from the lateral walls of 
the cranium. These lobes are expansions of the hypostomal parts of the 
subgenal margin of the cranium, and each is separated from the corres- 
ponding postgenal region by a distinct hypostomal suture Qis), which 
extends posteriorly to the root of the posterior arm of the tentorium 
concealed in the deep inflection at the base of the neck. The hypostomal 
lobes vary in size in different groups of caterpillars, but they do not 
unite to form a complete hypostomal bridge. 

A structural condition in the back of the head very similar to that 
of the caterpillars is found again in adult Tenthredinidae, which leads 
to a further specialization in the higher Hymenoptera. In Pteronidea, 
for example, the labium is displaced ventrally and united with the 
maxillae, and the base of the labiomaxillary complex is separated from 
the foramen magnum by two approximated hypostomal lobes of the 
cranial wall. In the adult of Vespa (Fig. 65 B) the hypostomal lobes 
are contiguous and united, forming thus a sclerotic bridge between the 
postgenal areas of the cranium (Pge), completely separating the foramen 



THE HEAD 


123 


magnum from the fossa containing the labium and maxillae. The 
hjrpostomal sutures are here absent, and the hypostomal lobes are con- 
tinuous with the postgenae. The evolution of the hypostomal bridge 
in the Hymenoptera reaches its final stage in the bees (C), where the line 
of union between the two hypostomal lobes is obliterated, and the 
posterior surface of the head presents a wide, continuous occipito- 
postgenal-hypostomal area in the center of which is the greatly reduced 
foramen magnum. A similar condition is found in the higher Diptera. 

It is important to note that in adult Hymenoptera and Diptera, as 
in the larvae of Lepidoptera, the posterior pits of the tentorium (Fig. 
55 B, C, pt) retain their primary positions in the lower parts of the post- 
occipital suture (pos) close to the foramen magnum and thus preserve in 
this respect the more primitive larval condition (A). 



Fig. 66. — Diagrams illustrating the hypognathous (A) and prognathous (B) types of head 

structure. 


The Gula . — The modifications in the posterior ventral parts of the 
head of insects in which a gula is developed are generally associated with 
a prognathous condition; the head, usually more or less flattened, is 
turned upward on the neck (Fig. 66 B), causing the true anterior surface 
to become dorsal, and the mouth parts to be directed forward. In 
insects of this type tbe under side of the head, which morphologically is 
the posterior surface, is lengthened by an expansion of the postgenal 
areas of the cranium; the bases of the maxillae lie far in advance of the 
foramen magnum, the hypostomal parts of the sufagenal sutures (hs) are 
extended, and the basal region of the labium is correspondingly 
elongated. In many prognathous insects, particularly in Coleoptera, 
a part of the ventral extension of the head lies posterior to the tentorial 
pits (pt), and in such cases the low'er ends of the postoccipital suture 
(pos), which terminate in the pits, appear to be dravni forward on tbe 
lower wall of the cranium. In the space between the ventral parts of 
the postoccipital suture, proximal to the tentorial pits, is formed the gula 
((?«). 


124 


PRINCIPLES OF INSECT MORPHOLOGY 


In studying the modifications of the head that have produced the 
gula of Coleoptera it is possible to start with forms that preserve the 
generalized or orthopteroid type of structure. The posterior surface 
of the head of a scolytid or scarabaeid larva (Fig. 67 A), for example, 
differs in no essential respect from that of Gryllus (Fig. 59 B). The 
labium of the scarabaeid larva is suspended from the neck membrane by a 
basal plate {Pmt), which is attached laterally to the postoccipital margins 
of the head (Poc) at points (a'", a'”) just behind the tentorial pits 



Fio. 67. — Heads of Coleoptera showing evolution of the gula. A, Popillia japonica 
larva; gula absent. B, Silpha larva; short gula (Gw) proximal to posterior tentorial pits 
(pO. C, Epicauta adult; gula typically developed. D, Scarites larva; gula obliterated, 
the gular area {gu) reduced to a median “suture.” 

{ft, ft) exactly as in the labium of the cricket. There is no sclerotization 
in the scarabaeid larva that might be regarded as a gula. 

The head of a silphid beetle larva (Fig. 67 B) has in general the same 
structure as that of the scarabaeid larva (A), though it is somewhat more 
elongate; but proximal to the tentorial pits {ft) there is here a short 
median sclerotic area, or gula {Gu), lying proximal to the basal plate 
of the labium {Smt) and continuous laterally with the postoccipital rim 
of the cranium behind the postoccipital suture {fos). 

With the elongation of the postgenal areas of the cranium proximal 
to the tentorial pits, the gular plate becomes correspondingly lengthened. 


THE HEAD 


125 


The characteristic structure of a coleopterous head having a well- 
developed gula is shown in the adult head of a meloid beetle (Fig. 67 C). 
The form of the cranium here differs from that of the scarabaeid or silphid 
larva principally in the lengthening of the postgenal regions to accom- 
modate the head to a more horizontal position. The general extension 
of the posterior part of the cranium has been accompanied by an elonga- 
tion of the gula (Gu), so that the tentorial pits {pt, pt) now lie a consider- 
able distance anterior to the foramen magnum, and the lower ends of the 
postoccipital suture (pos) appear to be drawn forward behind the pits. 

In certain coleopterous larvae the region occupied by the gula of the 
adult insect is entirely membranous (Fig. 160 A, gu), being merely 



Fig. 68. — Types of gular and labial structures in Coleoptera and Neuroptera. A, 
Staphylinus cinnamopterus adult: gula narrow; postmentum subdivided into mentum 
(Mt) and submentum (Smt); prementum (Prmt) simple. B, Corydalus larva: gula broad; 
postmentum (Pmt) undivided; prementum {Prmt) subdivided. 


an extension of the neck membrane in a ventral emargination of the head 
wall extending forward to the base of the labium. This region becomes 
sclerotized in the adult as a gular plate (B, Gu) continuous proximally 
with the postoccipital rim of the foramen magnum (Poc), anddistally 
with the basal plate of the labium (Smt). Since the gula seldom has a 
median suture, it is apparently formed in most cases by a uniform sclero- 
tization of the region primarily belonging to the neck between the lower 
ends of the postoccipital margins proximal to the tentorial pits. The 
parts of the postoccipital suture separating the gula from the postgenae 
are commonly termed the “gular sutures.” The gula varies in length 
according to the position of the tentorial pits, and it is sometimes nar- 
rowed, or almost obliterated, by a median approximation of the postgenal 
margins of the cranium (Fig. 68 A). A well-developed gula occurs also 
in some Neuroptera (Fig. 68 B). 






126 


PRINCIPLES OF INSECT MORPHOLOGY 


In many Coleoptera, especially in larval forms, there is no gula though 
the head may be elongate and the posterior tentorial pits may have an 
anterior position on the ventral side of the head. We have just noted, 
for example, that the gular area of the adult (Fig. 160 B) may be repre- 
sented in the larva (A) by a wide membranous area of the neck (gu) 
proximal to the tentorial pits. In the larvae of Scarites (Fig. 67 D), 
Thino'pinus (Fig. 160 C), Siaphylinus (D), and many other beetles, the 
gular region is narrowed to a median line or “suture” (gu) between the 
approximated postgenal areas of the cranium. 

Among the Isoptera the head of the soldier caste is often greatly 
lengthened posteriorly to accommodate the muscles of the huge mandibles 
(Fig. 69 A). The single proximal plate of the labium {Pmt) is in such 



Fig. 69. — Head of Termopsis, soldier form. A, under surface of head showing long 
postmentum bounded by elongate posterior tentorial pits (pi). B, cross section of head 
showing tentorium. C, diagram of tentorium, seen from behind, showing posterior arms 
(PT) united to form the roof of a long ventral channel above the postmentum {Pmt). 


cases correspondingly lengthened between the long postgenal regions of 
the cranium (Pge) and is sometimes called the “gula.” The apparent 
“gular sutures” {pt, pt) separating it from the postgenae, however, 
are found by examining the interior of the head (B) to be the lines of 
inflections that form the broad, plate-like posterior arms of the tentorium 
(B, C, PT). In the termite soldier, therefore, the grooves at the sides of 
the “gular” region of the labium (A,B, C, pt, pt) are the greatly lengthened 
posterior tentorial pits, and the structure of this part of the termite 
head, while similar to that in the Coleoptera, is not identical with the 
usual gular structure in this order, though in some of the Scarabaeidae the 
gula is likewise demarked by the greatly elongate tentorial pits (Fig. 159 
D, pt). 



THE HEAD 


127 


GLOSSARY OF TERMS APPLIED TO THE HEAD 

Antennal Sclerite (AaSc). — The sclerotic rim of the antennal socket within the 
antennal suture. 

Antennal Suture (as). — The line of inflection in the cranial wall surrounding the 
antennal socket. 

Cibarium (Cb). — The food pocket of the extraoral or preoral mouth cavity between 
the base of the hypopharynx and the under surface of the clypeus. 

Clypeus (Clp). — The facial area of the cranium just above the labrum, usually 
separated from the frons by an epistomal suture, and sometimes divided into an 
anteclypeus {AClp) and a postclypeus {Pclp)] the dilator muscles of the cibarium are 
attached on its inner surface. 

Coronal, or Metopic, Suture (cs). — The median dorsal arm of the epicranial suture. 

Cranium. — The sclerotic, skull-like part of the head. 

Epicranial Suture. — The dorsal Y-shaped suture of the cranium, including the 
median coronal suture (cs) of the vertex, and the divergent frontal sutures (fs) of the 
facial region. 

Epicranium. — A term variously applied to the entire cranium, to the cranium 
exclusive of the frons, or preferably to the upper part of the cranium. 

Epipharynx (Bphy). — A median lobe sometimes present on the posterior (or 
ventral) surface of the labrum or clypeus. 

Epistomal Suture (es).— The frontoclypeal suture; a groove uniting the anterior 
ends of the subgenal sutures across the face, forming internally a strong epistomal 
ridge (ER), typically straight, but often arched upward, sometimes absent. 

Foramen Magnum (For). — The opening from the head into the neck, usually 
called the occipital foramen. 

Frons (Fr ). — ^The facial area of the cranium between the frontal and epistomal 
sutures, or the corresponding area when the sutures are absent, bearing the median 
ocellus and the origins of the labral muscles. 

Frontal Sutures (fs). — The arms of the epicranial suture diverging ventrally (or 
anteriorly) from the coronal suture between the antennal bases toward the anterior 
articulations of the mandibles. 

Fulturae.^ — See suspensorium of hypopharynx. 

Genae (Ge ). — The lateral parts of the parietals, generally the areas behind and 
beneath the eyes. 

Gnathocephalon (Gnc).— The part of the head formed by the gnathal segments, 
bearing the mandibles and the first and second maxillae. 

Gula (Gu).- — A median ventral plate of the head in some prognathous insects 
formed by a sclerotization of the neck region proximal to the posterior tentorial pits, 
continuous with the postmentum or submentum. 

Gular Sutures. — The ventral ends of the postoccipital suture extended forward on 
the under side of the head in some prognathous insects. 

Hypopharynx (Hphy). — The median postoral lobe of the ventral wall of the 
gnathal region of the head anterior to the labium. 

Hypostoma (Hst). — The part of the subgenal margin of the cranium posterior to 
the mandible, usually narrow, but sometimes extended mesally as a hypostomal plate 
or as a hypostomal bridge in the ventral wall of the head. 

Hypostomal Suture (hs). — The part of the subgenal suture posterior to the mandi- 
ble, often obsolete or suppressed. 

Labrum (Lm). — The preoral lobe of the head suspended from the clypeus, with 
muscles arising on the frons. 



128 


PRINCIPLES OF INSECT MORPHOLOGY 


Mouth (Mth). — The external opening of the stomodaeum, situated in the ventral 
wall of the head between the labruna and the hypopharynx. 

Occipital Arch. — The area of the cranium between the occipital and postoccipital 
sutures; its dorsal part is the occiput proper (Oc), its lateral parts the postgenae (Pge). 

Occipital Condyles (occ).- — Processes on the margin of the postocciput to which 
the lateral neck plates are articulated. 

Occipital Suture (ocs). — A transverse groove sometimes present on the back of the 
head ending ventrally anterior to the posterior articulations of the mandibles. 

Occiput (Oc). — The dorsal part of the occipital arch, or also the entire arch includ- 
ing the postgenae. 

Ocular Sclerite (OSc). — A narrowband of the cranial wall encircling the compound 
eye within the ocular suture. 

Ocular Sutoe (os). — The line of inflection in the cranial wall around the com- 
pound eye, forming internally a circumocular ridge. 

Parietals (Prtl). — -The lateral areas of the cranium between the frontal and occipital 
areas, separated above by the coronal suture. 

Peristome. — The ventral marginal part of the cranium formed by the clypeus, or 
epistoma, the pleurostomata, and the hypostomata. 

Pleurostoma (PZs<). — The subgenal margin of the cranium bordering the mandible. 

Pleurostomal Suture (ps). — The part of the subgenal suture above the mandihle. 

Postfrontal Sutures (p/s). — Facial sutures present in some insects diverging from 
the coronal suture later ad of the antennal bases. 

Postgenae {Pge). — The lateral and ventral parts of the occipital arch, or areas of 
the cranium posterior to the genae. 

Postoccipital Suture (pos). — The posterior submarginal groove of the cranium 
having the posterior tentorial pits in its lower ends; internally it forms the postoccipital 
ridge {PoR) on which are attached the dorsal prothoracic and neck muscles of the head. 

Postocciput {Poc). — ^The extreme posterior rim of the cranium behind the post- 
occipital suture, probably a sclerotic remnant of the labial somite. 

Procephalon {Prc). — The region of the definitive head anterior to that of the 
gnathal segments, representing the primitive protocephalon, formed in the embryo 
from the cephahc lobes and the tritocerebral segment; bearing the labrum, the mouth, 
the eyes, the first antennae, and the second antennae when the last are present. 

Ptilinal Suture. — The crescentic groove cutting across the frons above the antennal 
bases in Diptera where the ptilinum has been withdrawn. 

Ptilinum. — An eversible sac of the frons in dipterous pupae used for rupturing the 
puparium. 

Salivarium {Slv). — The pocket between the base of the hypopharynx and the 
labium into which opens the salivary duct; in higher insects converted into a salivary 
pump or a spinning apparatus. 

Subgenal Areas {Sge). — ^The usually narrow lateral marginal areas of the cranium 
set off by the subgenal sutures above the gnathal appendages, including the pleuro- 
stomata and hypostomata. 

Subgenal Sutures {sgs). — The lateral submarginal grooves of the cranium just 
above the bases of the gnathal appendages, forming internally a subgenal ridge {SgR) 
on each side, continuous anteriorly with the epistomal suture when the latter is 
present. 

Suspensorium of the Hypopharynx. — A pair of bars or groups of sclerites in the 
lateral walls of the adoral surface of the base of the hypopharynx. (Fulturae.) 

Tentorial Pits. — The external depressions in the cranial wall at the roots of the 
tentorial arms; the anterior tentorial pits {at) located in the subgenal sutures or usually 



THE HEAD 


129 


in the epistomal suture, the posterior tentorial pits (pt) in the lower ends of the post- 
occipital suture. 

Tentorium (Tnt ). — The endoskeletal brace of the cranium formed of united 
anterior and posterior pairs of arms, bearing primarily the origins of the ventral 
muscles of the gnathal appendages, and usually giving attachment secondarily, on a 
pair of dorsal arms, to the antennal muscles. 

Vertex (T®). — The top of the cranium between and behind the compound eyes. 



CHAPTER VII 


THE HEAD APPENDAGES 

The usual appendages of the insect head include a pair of antennae, 
a pair of mandibles, a pair of maxillae, and the labium, the last representing 
a pair of united second maxillae. In the embryo of the walkingstick 
insect, however, there has been observed a pair of lobes l 3 dng anterior to 
the antennae, which possibly are rudiments of a pair of preantennae, 
and in the embryonic stages of various insects there are rudiments of 
postantennal appendages. The series of cephalic appendages appears to 
be the same in Crustacea and Myriapoda as in insects, except that the 
postantennal appendages are usually highly developed as the second 
antennae in Crustacea and appear to be entirely absent, even as embry- 
onic vestiges, in Chilopoda. The homologies of the head appendages in 
the several mandibulate groups are established by their innervation from 
corresponding cerebral and postcerebral nerve centers. In many 
Crustacea a pair of ventral head lobes known as the paragnatha occurs 
between the mandibles and the first maxillae, and a similar pair of lobes, 
the superlinguae, is present in some insects as lateral parts of the hypo- 
pharynx. Since the superlinguae of insects have been supposed to 
represent a pair of head appendages, they will be discussed in the present 
chapter, though it now seems probable that neither the paragnatha nor 
the superlinguae have the status of segmental limbs. 

Associated with the head appendages is a series of paired glands, 
which appropriately may be described in connection with the appendages. 

1. PREANTEWNAL APPENDAGES 

The existence of preantennal appendages in the Arthropoda cannot 
as yet be regarded as established. Heymons (1901), however, has 
described and figured a pair of evanescent appendage-like lobes in the 
embryo of Scolopendra lying anterior to the antennae (Fig. 70 A, Prnt), 
and Wiesmann (1926) reports the presence of a pair of similar preantennal 
rudiments in the embryo of a phasmid insect, Dixippus morosus (B, Prnt). 
The stalks bearing the compound eyes in certain Crustacea have an 
appendage-like structure, since they are movable, segmented, and amply 
provided with muscles; but since the compound eyes certainly belong to 
the prostomium, it seems most probable that the crustacean eye stalks are 

130 



THE HEAD APPENDAGES 


131 


of the nature of the sensory tentacular organs of the prostomium in 
Annelida. 




B 


Fig. 70. — Examples of the presence of apparent rudiments of preantennal appendages 
(Prnt) in arthropod embryos. A, Scolopendra. {From Heymons, 1901.) B, Dixippus 
(Carausius) morosus. {From Wiesmann, 1926.) 


2. THE ANTENNAE 

The antennae are the first of the appendicular organs of the head 
present in the adult insect. They are innervated from the deutocerebral 
lobes of the brain and generally have been regarded as the appendages of a 
corresponding antennal segment. Neither in their segmentation nor in 
their musculature, however, do the insect antennae resemble the limbs of 
the postoral somites, and the homologous organs of the Crustacea, the 
first antennae, or antennules, are never biramous in the manner character- 
istic of the second antennae and the succeeding appendages. If the 
antennae are not true segmental limbs, they must be regarded as organs 
analogous to the prostomial tentacles of the annelid worms. Though in 
the embryos of some of the lower insects the antennal rudiments arise at 
the sides of the mouth, or even behind the latter, the morphologically 
preoral position of their nerve centers in the brain suggests that the 
antennae belong to the preoral part of the head. In adult insects the 
antennae are situated on the anterior parts of the parietal regions of 
the cranium, usually on the facial aspect, but in many larvae and in some 
adults they are placed laterally just above the bases of the mandibles. 
Antennae are absent in the Protura, and they are practically absent in 
most larvae of the higher Hymenoptera, where the position of each is 
indicated only by a disc or a slight swelling over the tip of the imaginal 
organ developing beneath the larval cuticula. 

The typical insect antenna is a many-jointed filament, but generally 
three principal parts may be distinguished in its shaft (Fig. 71 A). The 
first part, by which the antenna is attached to the head, is usually 
larger than the others and constitutes a basal stalk of the appendage, 
termed the scape (Sep). The second part, or pedicel (Pdc), is short and in 
nearly all insects contains a special sensory apparatus known as the organ 


132 


PRINCIPLES OF INSECT MORPHOLOGY 


of Johnston. The part of the antenna beyond the pedicel is the flagellum, 
or clavola (FI). The flagellum is usually long and made up of many small 
subsegments, but it may be abbreviated or reduced to a single piece. 
Since the flagellar divisions in orthopteroid insects increase in number 
from one instar to the next, they appear to be secondary subdivisions of 
one primary antennal segment. The antennae are subject to many 
variations in form, giving rise to the several distinct types recognized in 
descriptive entomology, but the basic structure of the appendages is 
remarkably uniform. 

The base of the antenna is set into a small membranous area of the 
head called the antennal socket. The rim of the socket is often strength- 
ened by an internal submarginal ridge formed by an external inflection, 
the antennal suture (Fig. 71 A, as). Usually a pivotlike process on the 



Fig. 71. — Structure of the antenna. A, diagram showing the typical segmentation 
of an insect antenna. B, head of a chilopod, Sculigera forceps, with antennal muscles 
arising on the cranium. as, antennal suture; FI, flagellum; n, articular point (antenni- 
fer); Pdc, pedicel; 8cp, scape. 


rim of the antennal socket (n) forms a special support and articular point 
for the base of the scape, allowing the antenna a free motion in all direc- 
tions. The pivot, or antennifer, is generally ventral but is not always so, 
and in some cases it is obsolete or absent. 

Each antenna is moved as a whole by muscles inserted on the base of 
the scape. The origin of the antennal muscles in adult pterygote insects 
is commonly on the dorsal or anterior arms of the tentorium, but in some 
larval insects the muscles arise on the walls of the cranium (Fig. 64 B), as 
they do in the chilopods (Fig. 71 B). The attachment of the antennal 
muscles on the tentorium, therefore, is probably a secondary condition 
resulting from a migration of the cranial ends of the muscles to the dorsal 
tentorial arms after the latter have made connections with the head wall. 
The pedicel and the flagellum together are moved by muscles arising in 
the scape and inserted on the base of the pedicel (Fig. 71 A), but the 
flagellum and its subsegments, so far as observed by the writer, are never 
provided with muscles in insects. 



THE HEAD APPENDAGES 


133 


3. THE POSTANTENNAL APPENDAGES 

The appendages of the tritocerebral segment of the arthropods 
undoubtedly belong to the series of true limbs, and it seems very probable 
that they represent the first pair of appendages in this series. Morpho- 
logically the postantennal appendages are postoral, since they are inner- 
vated from the tritocerebral lobes of the brain, which are unquestionably 
postoral ganglia, since they preserve their ventral connections by a 
substomodaeal commissure. The postantennal appendages occur as 
functional adult organs only in Chelicerata and Crustacea. In the first 
group they form the chelicerae (Fig. 72 A) ; in the second they are the 
highly developed second antennae (B), which have distinctly the biramous 
leg type of structure, segmentation, and musculature. 



Fig. 72. — Types of functional postantennal appendages. A, chelicera of a scorpion. 
B, second antenna of a decapod crustacean, Spironloeharis, with biramous structure like 
that of following appendages. 

The postantennal appendages are at best rudimentary in all insects. 
Embryonic vestiges of them occur in representatives of several orders 
and are usually called “second antennal,” “intercalary,” or “premandib- 
ular” appendages (Fig. 23 B, Pnt). In a few adult insects {Campodea, 
Machilis heteropus, Dissosteira) small lobes have been observed before the 
mandibles, which may possibly be persisting rudiments of the postantennal 
appendages (Fig. 60 B, 1). The occurrence of corresponding structures 
has not been recorded in any stage of the myriapods. We have no 
evidence to suggest what the form of the postantennal appendages may 
have been when they were functional organs in insects, but it is perhaps 
reasonable to suppose that in terrestrial arthropods, in which the gnathal 
appendages were not yet added to the head, the tritocerebral appendages 
served in some capacity connected with feeding. 

4. THE MANDIBLES 

The mandibles are the appendages of the first gnathal segment and 
are undoubtedly homologous organs in all the mandibulate arthropods. 
The corresponding appendages of the Chelicerata are the pedipalps. 

The typical mandible of pterygote insects is a strong biting jaw 
hinged to the head by anterior and posterior articulations, and having a 
transverse movement of abduction and adduction produced by abductor 



134 


PRINCIPLES OF INSECT MORPHOLOGY 


and adductor muscles arising on the dorsal wall of the cranium. In 
most of the apterygote insects, however, and in the Mandibulata gen- 
erally, the mandible has a single point of articulation, and dorsal and 
ventral muscles, suggesting that it has been evolved from the basis 
of a leglike appendage provided with the usual tergal and sternal pro- 
moter and remoter muscles. The presence of a well-developed telopodite 
in the form of a segmented palpus on the mandible of many Crustacea, 
and the limblike structure of the corresponding appendage in certain 
Arachnida amply confirm the leg origin of the arthropod jaw. 

To understand the more specialized, though simpler, mandible of the 
Pterygota, we must first study the structure and musculature of the 
organ as found in apterygote insects and in other mandibulate arthropods. 
The leglike form of the appendage is well shown in the pedipalp of a 



Fig. 73. — Mandibles and mandibular musculature of Myriapoda. A, right mandible 
of a diplopod, anterior view. B, left mandible of Scutigera, anterior view. C, right 
mandible of same, dorsal view. 

phalangid (Fig. 47, B), which consists of a basis {Cxpd) provided with a 
large endite lobe (Bnd), and a long telopodite of six segments that are 
clearly identical with the segments of the legs. Among the Mandibulata, 
the mandible appears to be in some respects most generalized in the 
Diplopoda, since in this group it has certain features suggestive of the 
structure of an insect maxilla. The jaw of the Chilopoda is evidently 
derived from an organ similar to the diplopod mandible. In the more 
generalized Crustacea and Hexapoda the appendage is more generalized 
in certain ways, though it is only in the Crustacea that it retains the 
telopodite; but in the higher forms of these groups the mandible presents 
numerous specializations in its structure, musculature, and mechanism. 

The Mandibles of Diplopoda. — ^The diplopod mandible (Fig. 73 A) 
consists of a large basal part (Cxpd) and a movable terminal lobe (Lc). 
The sclerotic wall of the basis is distinctly divided into a proximal plate, 
or cardo (Cd), and a distal plate, or stipes {St). The musculature of the 
basis consists of two groups of fibers. The fibers of one gi’oup form a 
single anterior muscle (7) arising dorsally on the head wall and inserted 
on the upper (anterior) margin of the stipes. The fibers of the second 


THE HEAD APPENDAGES 


135 


group (EIL) form numerous muscle bundles connected with each mandi- 
ble, those from opposite sides taking their origins on a thick median liga- 
ment ( 2 ) from which they diverge into the cardo and stipes of each mandi- 
ble, the two sets forming thus a strong zygomatic adductor between the 
two jaws. The median ligament is supported from the dorsal wall of 
the head by two large vertical muscles. 

The movable distal lobe of the diplopod mandible (Fig. 73 A, Lc) 
is of particular interest because of its resemblance to the lacinia of an 
insect maxilla (Fig. 78, Lc). The lobe is hinged to the inner distal angle 
of the stipes and is provided with a short stipital flexor muscle {jlcs) 
arising within the stipes, and with a large cranial flexor (flee) arising on 
the head wall and inserted on a strong apodeme of the inner basal angle 
of the lobe. 

The Mandibles of Chilopoda. — The chilopod mandible (Fig. 73 B, C) 
is similar to the jaw of the diplopod, but the basis (B, Cxpd) is not sub- 
divided, and the distal lobe (Lc) is less movable, since it has no true 
articulation vnth the basis, though it is flexible on the latter and is pro- 
vided with both stipital and cranial flexor muscles (fles, flee). The basis 
is rotated on its long axis by an anterior dorsal muscle (I) and a posterior 
dorsal muscle (J); and it is provided with a ventral adductor (KL), the 
fibers of which take their origin on a median ligament supported on a 
pair of ventral apophyses arising at the base of the hypopharynx (Fig. 
61 A, HA). 

The Mandibles of Crustacea. — The crustacean mandibles present a 
great variety of forms, with many types of mechanism resulting from the 
different ways in which the organs are articulated to the head or the 
mandibular segment. In the more generalized groups of both the Ento- 
mostraca and the Malacostraca, however, they have a type of structure 
very similar to that of the chilopod jaws. The mandibles of Apus and 
Anaspides are good representatives of this apparently generalized type 
of structure. 

The mandible of Anaspides (Fig. 74 A) consists of an elongate basis 
(Cxpd) with a large endite lobe (End) and of a small three-segmented 
telopodite, or palpus (Tlpd). The basis is broadly implanted by its 
entire inner surface on the membranous lateral waU of the mandibular 
segment and is provided with a single dorsal point of articulation (a') with 
the tergum. The broad terminal lobe (End) is entirely immovable on 
the basis, but it is differentiated into incisor and molar areas. The 
musculature of the Anaspides mandibles is very simple. Each jaw is 
provided with an anterior rotator muscle (I) and a posterior rotator (J), 
both arising on the dorsal wall of the mandibular segment, and vuth 
strong ventral adductor muscles (KL). The fibers of the adductors are 
separated into two groups; those of one group take their origin on a 



136 


PRINCIPLES OF INSECT MORPHOLOGY 


median ligament (fc) arising from the ventral body wall, those of the other 
group are continuous from one jaw to the other and form a zygomatic 
adductor. 

The mandibles of Apus are similar to those of Anaspides, but they 
lack palpi, and the adductor apparatus consists only of a large, dumb- 
bell-shaped zygomatic muscle between the two jaws. This same type of 
mandible is found in many other crustaceans and is evidently the one 
from which the more specialized types have been evolved. 

The Mandibles of Apterygote Insects. — Among the Apterygota the 
mandibles resemble those of the Chilopoda and the simpler Crustacea in 
all groups except Lepismatidae, in which they take on special features 
characteristic of the mandibles of Pterygota. In all insects the man- 
dibular telopodite is entirely absent. 



Fig. 74. — Generalized type of mandible in Crustacea and Hexapoda. A, Anaspides 
tasmaniae, posterior view. B, Heterojapyx gallardi, dorsal view. C, Nesomachilis maon- 
cus, posterior view. 

In the Japygidae the mandibles are deeply retracted into the head, 
but each consists of a slender basis (Fig. 74, B, Cxpd) articulated to the 
head by a single point of articulation {a'), and ending distally in an elon- 
gate terminal lobe (Bnd). The appendage is provided with anterior and 
posterior rotator muscles (7, J) arising on the dorsal wall of the head and 
is equipped with two ventral adductor muscles {KL). The fibers of one 
pair of adductor muscles from the two mandibles (KLt) arise medially 
on a pair of sternal apophyses of the head springing from the base of the 
hypopharynx (Fig. 61 B, HA). These apophyses, as we have seen 
(page 118), are evidently the prototypes of the anterior tentorial arms in 
the Pterygota. The fibers of the other pair of muscles (KLz) from the 
opposite jaws are united upon a median ligament ( 2 ) and constitute a 
common zygomatic adductor. 

In the Machilidae the mandibles (Fig. 74 C) are exserted, but they 
have the same essential structure as in Japygidae. The free distal lobe 
of each is differentiated into a slender incisor point and a thick molar 
process. The muscle fibers of the adductor apparatus are disposed in 
two distinct groups, those of one group {KLt) forming a wide fiat muscle 


THE HEAD APPENDAGES 


137 


attached medially on the corresponding hypopharyngeal apophysis (HA), 
those of the other (KLz) converging upon a narrow median ligament (z) 
to form, with the corresponding group from the opposite jaw, a zygomatic 
adductor between the two mandibles. 

Morphology of the Arthropod Mandibles. — From the brief review 
just given of the basic structure of the mandibles in the principal groups 
of mandibulate arthropods, it is evident that the mandible has been 
evolved from a limb of the ambulatory type, and that the modifications 
that have produced the more generalized forms of the jaw are of a com- 
paratively simple nature. 

The body of the mandible corresponds to the coxopodite of a general- 
ized appendage; the telopodite is retained as a palpus in many of the 
Crustacea, but in other groups it has been completely suppressed. The 
projecting terminal lobe is an endite of the basis; in the Diplopoda this 
lobe is freely movable, and in both the diplopods and the chilopods it is 
provided with muscles corresponding to the muscles of the lacinia of a 
generalized insect maxilla. In other groups the terminal lobe loses its 
mobility and becomes solidly fused with the basis, in consequence of 
which its muscles have disappeared. The anterior and posterior dorsal 
muscles of the mandibular base correspond to the dorsal promotor and 
dorsal remotor of a generalized appendage (Fig. 41, 7, J). The ventral 
muscles evidently represent the ventral promoters and remotors (K, L), 
which, being grouped together, become functionally the adductors (KL) 
of the generalized mandible. The somatic ends of the adductors are 
usually supported on sternal apophyses of the ventral head wall or on a 
membranous fold of the latter or of the apophyses. The membrane 
between the ends of some of the fibers from opposite jaws may become 
detached and form a ligament uniting the opposing fibers, thus producing 
a zygomatic adductor between the mandibles having no connection with 
the body wall. 

The Mandibles of Pterygote Insects. — The typical mandible of the 
biting type in pterygote insects is quite different in both its mechanism 
and musculature from the mandible of most Apterygota and other arthro- 
pods. The pterygote type of mandible, however, is found in Lepis- 
matidae, and a similar form of mandibular mechanism has been developed 
in some of the higher Crustacea. 

The jaw of the more generalized mandibulate arthropods, as we have 
seen, is hinged to the lower edge of the cranium, or the mandibular seg- 
ment of the body, by a single point of articulation (Fig. 75 A, a'), which 
evidently corresponds to the dorsal articulation of the basis of an ambula- 
tory limb (Fig. 43 A, B, a). The mandible of Lepismatidae and Ptery- 
gota differs from a generalized mandible in that it has a secondary anterior 
dorsal articulation on the head (Fig. 75 B, c) and thus acquires a long 



138 


PRINCIPLES OF INSECT MORPHOLOGY 


axis of attachment (a'-c) with a definitely limited transverse movement of 
abduction and adduction. By this change from a pivotal to a dicondylic 
hinge articulation, the primitively anterior and posterior dorsal muscles 
of the jaw {A, I, J) become respectively a dorsal abductor (B, I) and a 
dorsal adductor (J). The ventral muscles remain unaltered in function, 
but, with the increasing size of the dorsal adductors, they become of 
secondary importance and are usually reduced (B, KL) or absent in 
higher forms. In many of the lower Pterygota and in Lepismatidae, 
however, the ventral adductors persist; they are highly developed in the 
larvae of Ephemerida and Odonata, where they arise on the anterior arms 
of the tentorium, and they are represented in the adult Isoptera and 
most Orthoptera, except Acrididae. 



Fig. 75. — Diagrams of types of insect mandibles. A, apterygote type with one articula- 

tion (o'). B, pterygote type with two articulations (o', c). 

The typical biting jaw of pterygote insects, therefore, by the acquisi- 
tion of a long hinge line on the head, with anterior and posterior articula- 
tions, comes to have a transverse movement of abduction and adduction 
(Fig. 75 B), and the primitive dorsal promotor and remotor muscles 
(7, J) come to be, respectively, dorsal abductors and adductors. The 
dorsal adductor increases in size, while the ventral adductors {KL) are 
reduced, and finally, with the disappearance of the ventral muscles, the 
adductor fimction is taken over entirely by the dorsal muscle. In addi- 
tion to these changes in the motor apparatus, the action of the mandible 
undergoes an alteration by a change in the slope of its axis. In the more 
primitive condition, retained in Lepismatidae, the axis of the jaw slopes 
downward from the posterior articulation to the anterior articulation; 
in most Pterygota it is oblique in the opposite direction, thus giving the 
tip of the jaw a posterior motion accompanying the movement of 
adduction. 

The structure and mechanism of the biting type of mandibles in the 
Pterygota are well represented by the mandibles of an acridid grass- 
hopper (Fig. 76). Each mandible (D) is a thick, strong appendage with 



THE HEAD APPENDAGES 


139 


a broad triangular base, having its mesal surface differentiated into a 
distal toothed incisor lobe (o) and a proximal molar lobe (p). The jaw is 
broadly hinged to the pleurostomal margin of the cranium by the outer 
edge of its triangular base and has a strong articulation with the head 
at each end of the hinge line (A, C, c, a'). It should be observed that the 
articular surfaces of the mandible lie outside the basal membranous 
connection of the jaw vdth the head; they are merely specialized points 
of contact between the mandible and the cranium (Fig. 27 D). The 
musculature of the grasshopper mandible consists only of a dorsal abduc- 
tor and a dorsal adductor. The abductor is relatively small; it arises on 



Fig. 76. — Endoskeletal structures of the cranium, and mandibles of a grasshopper. 
Dissosteira Carolina. A, interior view of right half of cranium. B, the tentorium, ventral 
view. C, same as A, but with clypeus, labrum, and right mandible and muscles in place. 
D, right mandible and its apodemes, posterior view. 

the lateral wall of the cranium and is inserted on a small ap'odeme (Fig. 
76 D, 8Ap) attached to a flange of the outer margin of the mandibular 
base sufficiently far outside the axis line to give effectiveness to the muscle. 
The adductor is a huge muscle composed of several bundles of fibers 
(C, 9a, 9&) arising on the dorsal and lateral walls of the cranium, and 
inserted on a large apodeme (D, 9Ap) attached at the inner angle of the 
mandibular base. The width of the mandible between the hinge line 
and the point of attachment of the adductor apodeme gives great power 
to the adductor muscle in closing the jaw. 

Further modifications of the pterygote mandible by which it becomes 
adapted to various specialized modes of feeding will be described in 
Chap. XII. 

6. THE SUPERLINGUAE 

The paired ventral lobes of the head known as the superlinguae 
(or “ paraglossae ”) are best developed in apterygote insects and in some 



140 


PRINCIPLES OF INSECT MORPHOLOGY 


of the lower members of the Pterygota. In adult insects the super- 
linguae, if discernible as such, always appear as lateral lobes of the 
hypopharynx (Fig. 77 C, D, Slin), the median part of which (Lin) is 
designated the lingua (or “glossa”)- In embryonic stages of apfcerygote 
insects, however, the superlinguae arise as independent lobes of the 
ventral wall of the head in the neighborhood of the mandibles (A, Slin), 
and for this reason they have been regarded by some writers as represent- 
ing a pair of postmandibular appendages, equivalent to the first maxillae 
(maxillulae) of Crustacea, that have secondarily united with the median 



Fig. 77. — The hypopharynx of insects and a crustacean. A, ventral view of head of 
embryo of Anurida marilima, showing rudiments of lingua {Lin) and superlinguae (Slin). 
(From Folsom, 1900.) B, embryonic superlinguae of Tomocerus. (From Hoffmann, 1911.) 
C, hypopharynx of ephemerid nymph. D, hjrpopharynx of Nesomachilis, posterior view. 
E, detached superlingua of same. F, hypopharynx of an isopod crustacean, composed of 
median lingua and lateral paragnatha (Pgn) , posterior view. 


lingua to form the definitive hypopharynx. The shape of the super- 
linguae in certain apterygote insects is somewhat suggestive of a rudi- 
mentary limb appendage (D, E), but in others the form is so variable that 
little significance can be attached to it in any case. The embryonic 
superlinguae of Collembola have been said to be innervated from special 
centers in the suboesophageal ganglion, but different claimants disagree 
as to the position of the alleged centers, and most investigators find 
no evidence of the presence either of such nerve centers or of a correspond- 
ing head somite. According to Hoffma nn (1911) the superlinguae of 
Tomocerus arise as lobes at the bases of the mandibles (B. Slin), and 
Silvestri (1933), in a study of the development of the head appendages 
of J apyx, shows conclusively that the superlinguae are formed in connec- 
tion with the median part of the hypopharynx as lobes of the mandibular 
somite. 



THE HEAD APPENDAGES 


141 


The superlinguae of some adult insects have a close resemblance to 
the paragnatha of certain Crustacea (Fig. 77 F, Pgn), and, when the 
paragnatha are united with a median lingua (Lin) , the resulting structure 
is very similar to the insect hypopharynx (C). The paragnatha are said 
to be innervated by branches of the mandibular nerve trunks, and there 
is no evidence that the organs are other than secondary lobes of the head. 
If the paragnatha and the superlinguae are not homologous, they are 
entirely analogous structures developed in crustaceans and insects, but 
not in the myriapods. 


6. THE MAXILLAE 

The maxilla of insects having typical biting mouth parts closely 
preserves the structure of a limb that may be supposed to have iwo 



Pig. 78. — Diagram of the structure and musculature of a typical insect maxilla, sug- 
gesting that the cardo and stipes represent the coxopodite (Cxpd) and the palpus the telo- 
podite pTlpd) of a leg. 

movable basal endites provided with muscles arising in the basis. The 
telopodite is relatively small and palpiform, but it has the essential 
structure of the shaft of a leg beyond the coxa both in its segmentation 
and in its basal musculature. 

General Structure of a Maxilla.— The maxillary basis is typically 
elongate (Fig. 78, Cxyd) and is implanted by its entire inner surface on 



142 


PRINCIPLES OF INSECT MORPHOLOGY 


the pleural region of the head (Fig. 54 B) just behind the mandible. On 
its dorsal extremity it bears a single condyle (Fig. 78, a") by which it 
articulates with the lower lateral margin of the tergal region of the 
cranium (T). A line of flexure in the upper part of the outer wall 
divides the basis into a proximal cardo {Cd) and a distal stipes (St). 
Usually the cardo is flexed mesally on the upper end of the stipes (Fig. 
80 A, C). At its ventral extremity the stipes bears two endite lobes. 
The mesal lobe is the lacinia (Fig. 78, Lc), the outer lobe the galea (Ga). 
Laterad of the galea arises the palpus (Pip), or telopodite. The relative 
size and the segmentation of the palpus vary much in different insects. 
In many orthopteroid forms there are two small segments in the base of 
the palpus which appear to be trochanters (Tr), the second followed by a 
longer femurlike segment (Fm), which is separated by a characteristic 
femoro-tibial joint (ft) with a ventral flexure from a distal part containing 
two or three segments. The proximal articulation of the palpus on the 
basis (ct) has a dorsoventral movement suggestive of that of the coxo- 
trochanteral joint of a leg. The region of the stipes supporting the palpus 
is sometimes differentiated as a palpifer, but the musculature of the 
palpus gives no reason for believing that the palpifer in any way repre- 
sents a primitive segment of the maxillary limb. 

The Muscles of the Maxilla. — ^The musculature of a typical maxilla 
in biting insects comprises muscles of the basis that move the appendage 
as a whole, muscles arising within the basis that move the terminal lobes 
and the palpus, and muscles of the palpus segments. 

The basal musculature of the maxilla in all biting pterygote insects 
(Fig. 78) is very similar to the musculature of the mandible in apterygote 
insects and other arthropods. It includes anterior and posterior dorsal 
muscles (I, J) taking their origins on the tergal wall of the head, and 
ventral adductors (KL) arising on the tentorium (Tnt) in pterygote 
insects, or on the hypopharyngeal apophyses in apterygote insects (Fig. 
79 B). The dorsal muscles may comprise an anterior rotator (Fig. 78, 
rtmxa) and a posterior rotator (J) attached on the cardo, though usually 
the second is absent; but nearly always there is a large anterior muscle 
(flee) inserted on the inner angle of the base of the lacinia, which is thus a 
cranial flexor of the lacinia, resembling in every way the similar muscle of 
the mandible in the diplopods and chilopods (Fig. 73, flee). The fibers of 
the ventral adductor muscles (Fig. 78, KLt), taking their origin on the 
tentorium (Tnt), are usually separated into two groups, those of one group 
being inserted in the cardo, those of the other on the posterior margin of 
the stipes. These muscles are here termed “adductors ” because morpho- 
logically they correspond to the ventral adductors of a primitive append- 
age ; in function, however, they may produce various movements of the 
maxilla. 



THE HEAD APPENDAGES 


143 


The muscles of the terminal lobes of the maxilla always have their 
origin in the stipes. A stipital flexor of the lacinia (Fig. 78, flcs) is inserted 
on the basis of the lacinia, and a flexor of the galea (fga) on the base of the 
galea. The lobe muscles may be branched, but they never occur in 
antagonistic pairs. 

The palpus is moved by a levator muscle (Fig. 78, 0) and a depressor 
(Q), both arising in the stipes, but never in the palpifer. These muscles 
are evidently homologues of the levator and depressor of the telopodite of 
a leg that arises in the coxa and are inserted on the trochanter; the 
depressor of the palpus, however, never has a branch corresponding to 
the body muscle of the leg trochanter. The segments of the palpus have 
usually each a single muscle inserted on its base (Fig. 80 C). 

Structural Variations of the Maxillae. — The maxillae may become 
variously reduced in different groups of insects, particularly in larval 



Fig. 79. — Maxilla of an apterygote insect, Heterojapyx gallardi. A, ventral (posterior) 

view. B, dorsal (anterior) view. 

forms, by a suppression of one or both of the terminal lobes or by the 
loss of the palpus; but other than this, except in the Hemiptera and 
Diptera and certain other piercing or sucking insects, they do not depart 
radically from the generalized type of structure, and they are funda- 
mentally alike in both Apterygota and Pterygota. The crustacean 
maxillae are in general almost rudimentary appendages by comparison 
with the typical insect maxilla, but a study of such forms as Anaspides 
brings out a fundamental similarity in the basal structure. The post- 
mandibular appendages of the Chilopoda are small leglike structures, each 
composed of a large basis and a reduced telopodite, but endite lobes may 
be present on the bases of the first pair. 



144 


PRINCIPLES OF INSECT MORPHOLOGY 


The apterygote maxilla is scarcely more primitive than that of 
the lower Pterygota. In the Thysanura the maxillae are suspended from 
the hypostomal margins of the cranium (Fig. 82 A, Mx) and have the 
usual maxillary structure. In Japygidae the maxillae are mostly con- 
cealed with the mandibles in deep pouches above the labium in which they 
have a horizontal position. The basis of each appendage, however, is 
composed of a small cardo (Fig. 79 A, Cd) and a long stipes {St). The 
stipes bears a short palpus (Pip), a strongly sclerotized lacinia (Lc), and a 
weak galea (Ga). The palpus and galea are somewhat separated from 
the stipes and connected with the base of the hypopharynx; this condition 
has given rise to the idea that they are not parts of the maxilla but repre- 
sent the superlinguae. The fact, however, that both structures are well 



Fig. 80. — Maxillae of Periplaneta americana. A, left maxilla, posterior view. B, inner 
surface of cardo. C, right maxilla, anterior view. 

provided with the usual muscles arising in the stipes (B) shows conclu- 
sively that they belong to the maxilla. The lacinia is equipped with a 
broad stipital flexor (flcs) and a large cranial flexor (flee) arising on the 
back of the head. The basal musculature of the appendage consists of 
an anterior dorsal muscle (I) and of two groups of ventral adductors 
(adcd, adst) arising on the hypopharyngeal apophysis (HA). 

The generalized structure of the pterygote maxilla is well exemplified 
in the maxilla of the cockroach (Fig. 80). The cardo and the stipes (A, 
Cd, St) are flexible on each other by a distinct hinge, and their planes form 
an abrupt angle at the union. The cardo is articulated to the cranium 
by a basal condyle (a") ; its surface is marked by the line of an internal 
ridge (B, r), but it is not otherwise “divided”as it is sometimes said to be. 
The elongate stipes has a sutural groove (A, q) near its inner edge which 
forms internally a submarginal ridge on which muscles are attached. In 
some insects the surface of the stipes is marked by other ridge-forming 



THE HEAD APPENDAGES 


145 


grooves or by sutures, but, as with the cardo, these features are not 
evidence that the stipes is a composite sclerite. 

The large terminal lobes of the cockroach maxilla arise from the 
distal end of the stipes, the galea (Ga) being external and the lacinia 
(Lc) mesal, but the galea also partly overlaps the lacinia anteriorly. 
The galea is a broad, soft lobe, widened distally; the lacinia is more 
strongly sclerotized and ends in an incisor point provided with two apical 
teeth curved inward, and its inner margin is fringed with long hairs. 
Both lobes are movable on the end of the stipes; the galea can be deflexed, 
the lacinia can be flexed mesally. The galea has a single muscle (C, fga) 
arising in the stipes. The lacinia has both a stipital flexor (flcs) and 
a cranial flexor (flee), but the two are united with each other at their 
insertion on the lacinia. In some insects the area of the stipes support- 
ing the galea is differentiated as a distinct lobe called the subgalea, 
but the base of the true galea is to be determined by the point of attach- 
ment of its muscle. 

The long maxillary palpus of the roach is composed of five segments 
(Fig. 80 A, B, Pip). There is no palpifer lobe differentiated in the stipes, 
and the small basal segment of the palpus cannot be mistaken for a 
palpifer, since the palpus muscles (C, Iplp, dplp) are inserted upon it. 
There are only three muscles within the palpus, the first being a muscle of 
the second trochanteral segment arising in the first trochanter, the second 
a long ventral muscle (T) of the tibial segment arising also in the first 
trochanter, and the third (F) a muscle of the terminal segment having its 
origin in the tibia. Between the third and fourth segments there is a 
characteristic femoro-tibial flexure (ft). 

The basal musculature of the roach maxilla (Fig. 80 C) is of the usual 
type. There is but a single anterior dorsal muscle (I) inserted on the 
cardo. The ventral muscles arising on the tentorium consist of two large 
groups of fibers, one (aded) inserted in the cardo, the other (adst) on the 
mesal ridge of the posterior surface of the stipes (A, q). These muscles, 
though they are evidently the primary sternal adductors of the append- 
age, give a movement of protraction to the maxilla because of the angula- 
tion between the cardo and stipes, and because the stipes rests and moves 
against the side of the hypopharynx. 

7. THE LABIUM 

The insect labium is a composite structure. Its major part is formed 
by the union of a pair of gnathal appendages closely resembling the 
maxillae, but the organ perhaps includes in its base a part of the sternal 
region of the labial somite. The component labial appendages are 
termed the seeond maxillae of insects, and there is little doubt that they 
correspond to the second maxillary appendages of Crustacea. In certain 



146 PRINCIPLES OF INSECT MORPHOLOGY 

Crustacea the first maxillipeds are united in a labiumlike organ attached 
to the head. 

In its generalized form, the labium (Fig. 81 A) consists of a fiat 
median part, of two lateral segmented palpi {Plv), and of four unseg- 
mented terminal lobes {Gl, Pgl). Structurally the organ is divisible into a 
free distal prelabium (Prlb) bearing the palpi and the terminal lobes, and a 
proximal postlabium {Plb) largely or entirely adnate on the posterior or 
ventral wall of the head. The line of flexibility between the two parts 
may be termed the labial suture (lbs). All the proximal muscles of the 
labium are inserted on the movable prelabium. 

The body of the prelabium is commonly termed the prementum (Prmt ) ; 
the postlabium, therefore, may be called the postmentum (Pmt). Some 
writers designate the two primary parts of the labium “mentum” and 



flcs-.W 



Fig. 81. — Diagrams illustrating the fundamental structure of the insect labium, and 
the correspondence of its parts with those of a pair of united maxillae. Plb, postlabium, 
consisting of the postmentum (.Pmt)\ Prlb, prelabium, including the prementum, or iabio- 
stipites IPrmt), palpi (Pip), and terminal lobes (Gl, Pgl). 

^‘submentum,” respectively, but more generally these names are given to 
subdivisions or sclerites of the postlabium. Unfortunately the current 
terms given to the parts of the labium cannot be made to fit consistently 
with the morphology of the organ. The prelabium functionally is the 
under lip of the insect, and it has been termed the “eulabium” by 
Crampton (1928); but commonly the name “labium” applies to the 
entire organ, and terms composed with “mentum” are given to its 
several parts or sclerites. Only by a radical change in the labial nomen- 
clature could its numerous inconsistencies be eliminated (see Walker, 
1931). 

The Prelabium. — The prelabium (Fig. 81 A, Prlb) is the movable 
distal part of the labium. It is composed of a central body, the pre- 
mentum (Prmt), of the labial palpi (Pip), and of the terminal lobes (Gl, 
Pgl), the last collectively constituting the ligula (Lig). 

The Prementum. — When the labium is compared with a pair of 
maxillae, it becomes evident that the prementum (Fig. 81 A, Prmt) repre- 





THE HEAD APPENDAGES 147 

sents the united stipites of a pair of maxilla-like appendages (B, St) , since 
it is the part of the labium in ^Yhich arise the muscles of the palpi and the 
ligular lobes. Tlie prementum, therefore, is appropriately designated 
pars stipitalis labii, or hhiostipifes. The paired origin of the prementum 
is suggested often by a distal cleft between its stipital components (Fig. 
S3) or b 3 '- the presence of paired sclerites in its ventral wall (Fig. 158 C). 
Lateral lobes of the prenientum bearing the palpi are frequently differ- 
entiated from the median area and are termed the palpigcrs (Fig. 81 A, 
Pig), since they are analogous with the palpifers of the ma.villae (B, Plf). 
The size of the prementum varies much in different groups of insects. 
In adult Coleoptcra, for example, it is often a relativelj'' small part of the 
labium (Figs. 67 C, 68 A, 158 C, 160 B, Print), while in the higher Hjnnen- 
optera it becomes the major piece of the appendage (Fig. 158 F), and in 

y? pos 


A 

Fig. S2. — Articulation of the Knntlinl appendages on the cranium. A, an apterygote 
insect, Ncsomacliilxs, with single mandibular (o'), maxillary (o"), and labial (a"’) articula- 
tion on each side. B, head of a myrmcloonid, posterior tdeiv. 

the prehensile labium of odonate larvae it is a large spatulate lobe (E) 
bearing the clawlike palpi {Pip) on its distal angles. There has been 
much confusion as to the identity of the prementum in comparative 
studies of the labium, but ati examination of the labial musculature will 
seldom fail to give a positive determination of the limits of the stipital 
region of the labium, which is the prementum. 

The muscles inserted on the prementum comprise two pairs of cranial 
adductors arising on the tentorium (Fig. 84 A, ladlh, 2adlb), and a pair 
of median retractors (or flexors) arising on the postmentum (Figs. 81 A, 
84 A, rst). The muscles that take their origin within the prementum 
include the muscles of the palpi {Iplp, dplp) and the muscles of the 
terminal lobes [jgl, fpgl) , together usually isuth muscles associated with the 
duct of the labial glands (Fig. 84 A, Is, 2s). 

The sclerotization of the ventral wall of the prementum is highly 
variable. Typically it forms a single premental plate (Figs. 59 B, 83 B, 
158 E, F, Prmt), but often it is broken up into two or more sclerites. 




148 


PRINCIPLES OF INSECT MORPHOLOGY 


In some adult Coleoptera there is present a pair of lateral premental 
sclerites (Fig. 158 C, Prmt), while in adult Neuroptera and many larval 
Coleoptera the premental sclerotization is characteristically subdivided 
into a distal plate or group of sclerites (Figs. 67 B, 68 B, 82 B, 158 A, B, 
159 A, B, ab, or a, b), giving insertion to the tentorial adductor muscles 
(Fig. 159 B), and into a proximal plate or pair of sclerites (c), on which 
are inserted the median retractor muscles {rst) from the postmentum. 
The surface of the prementum, again, is sometimes entirely membranous. 

The Labial Palpi. — The palpi of the labium are usually shorter than 
the maxillary palpi and are commonly three segmented (Fig. 83) . Each 
is pro^dded with levator and depressor muscles taking their origins in 
the prementum (Figs. 81 A, 83 B, 84 A, Iplp, dplp). Generally, therefore, 
the palpi can be distinguished from the terminal lobes of the labium by 
their provision vdth antagonistic muscles. In some cases, as in odonate 
larvae, and possibly in adult Diptera, the ligular lobes are suppressed 
and the palpi become the movable terminal appendages of the labium 
(Fig. 158 E, Pip). 

The Ligula. — The terminal lobes of the labium vary much in relative 
size and shape in different insects (Fig. 83) and are sometimes subdivided 
(A); rarely they are absent, but they may be variously united. Col- 
lectively the lobes (or the distal part of the labium including the lobes) 
constitute the ligula (Fig. 81 A, Lig). 

The labium typically has four terminal lobes borne on the distal 
margin of the prementum (Fig. 81 A). The median pair are the glossae 
{Gl) , the lateral pair the paraglossae (PgT) . The labial lobes are clearly the 
laciniae and galeae of the united labial appendages (B, Lc, Ga). They 
have the same type of musculature as the lobes of the maxillae, each 
being provided with a single or branched flexor arising in the prementum 
(Figs. 81 A, 83 B, 84 A), but never having a pair of antagonistic muscles. 
The ligular lobes are sometimes confluent at their bases, sometimes the 
pair on each side are united, or, again, the two glossae are combined to 
form a single median lobe (Fig. 158 D, Gl), which, as in the bees, may be 
prolonged in a slender tongue-like organ (F, Gl), and, finally, the four 
lobes may be fused in a single ligular flap terminating the labium (Figs. 
82 B, 158 B, Lig). On the other hand, the labial lobes are often reduced, 
and one or both pairs may be absent. In many hoiometabolous larvae 
having the hypopharynx adnate upon the anterior (or dorsal) surface of 
the prementum, the ligula is fused with the end of the hypopharynx in a 
composite terminal lobe bearing the orifice of the labial glands at its 
extremity (Figs. 161 B, 164 C). 

The Postlabium. — Since the postlabium (Fig. 81 A, Plb) has no appen- 
dicular parts, it consists entirely of the proximal region of the body 
of the labium. To preserve uniformity in the nomenclature of the labial 



THE HEAD APPENDAGES 


149 


regions, therefore, the surface of the postiabium, as distingvhshed from the 
distal prementum of tlic prclabium, may be termed the postmenhim 
(Pint). The postlabial sclerotizatioii is so variable that the limits of the 
postmentum cannot be determined by a study of the labial sclcrites alone. 
In practice the postmentum is to be identified as that part of the labium 
l 5 dng proximal to the wscrliorts of all the labial muscles. The only 
muscles having their origins on tlic postmentum are the median muscles 
of the labium (rst) that extend from the postmentum to the prementum. 



Flo. 83. — Tj'pes of Rcncrnlizcd structure of the Inbium. A, ^^achi^{s. B, Termopsis, 
soldier form: body of Inbium composed of prementum (Prml) and postmentum (Pml) only. 
C, Biotin orientalis, with small mental sclerites (Ml) in distal part of postmontal region. D, 
Scudderia, wth well-developed mentum (Afl) and submentum (B/nt). 

Though the postmentum is usually broadly adnate on the posterior, or 
ventral, tvall of the head, its distal part sometimes projects to give 
support to the movable prelabium. In the larvae of Odonata the basal 
stalk of the labium (Fig. 158 E) appears superficially to be the post- 
mentum, but the musculature and mechanism of the organ are here so 
different from those in a labium of typical structure that the homologies 
of its parts become questionable. A recent paper by Munscheid (1933) 
gives a good account of the musculature of the odonate labium, but the 
mechanism of the larval organ is not satisfactorily explained. 

In the more generalized insects, the labium joins the neck membrane 
on a line between the posterior tentorial pits (Figs. 59 B, 82 A), and the 
proximal angles of its basal plate (a'") are attached to the postoccipital 



150 


PRINCIPLES OF INSECT MORPHOLOGY 


rim of the head just behind or below the tentorial pits, in line with the 
articulations of the maxillae and mandibles on the subgenal margins of 
the cranium (a", a'). The postmen turn thus corresponds in position 
to the cardines of the maxillae and would appear, therefore, to include in 
its composition the cardines of the primitive labial appendages (Fig. 81 B, 
Cd). Since, however, the secondary median orifice of the labial glands, 
which belongs to the venter of the labial somite, moves forward during 
development until it comes to lie anterior (or dorsal) to the bases of the 
labial appendages (Fig. 155, SIO), it seems probable that the postmentum 
contains in its median part also an element derived from the venter of the 
labial segment. The postmentum thus may be regarded as a composite 
structure formed by the union of the cardinal parts of the labial append- 
ages with the primitive sternum of the labial segment, in which char- 
acter it would resemble the definitive sterna of most of the succeeding 
body segments. Some writers regard the entire postmentum as a sternal 
derivative, but in this case it must be assumed that the labial cardines are 
absent, and that the sternum of the labial segment has become interposed 
between the tergum and the appendages in such a way that the latter are 
supported by the sternum alone. In the Machilidae the lateral areas of 
the postmentum (Fig. 83 A, Cd) are separated by faint lines from a 
triangular median area {stn) in a manner suggestive that the postmentum 
{Pmi) has a cardinosternal composition. 

The sclerotization of the postlabium forms in many insects a single 
postmental plate (Fig. 81 A, Pmt). This plate may cover the entire area 
of the postmentum, as in Apterygota (Fig. 83 A), termites (B), and some 
Neuroptera (Fig. 158 B), though again it may occupy only the basal part 
of the postmentum (Fig. 82 B), or, as in most caterpillars, it may be 
reduced to a small sclerite (Fig. 164 C, 'pmt) in the otherwise membranous 
postmental wall. On the other hand, the postmental area of the labium 
may be entirely membranous, as in hymenopterous larvae (Fig. 161, B, 
D, F, Pmt). 

In the majority of Orthoptera and adult Coleoptera the postlabial 
aiea, or postmentum, contains two distinct plates. The distal plate in 
such cases is generally called the mentum (Figs. 59 B, 67 C, 83 D, Mt); the 
proximal one the submentum (Smt). The mentum always lies proximal to 
the insertions of the median muscles of the prementum, which arise on 
the submentum when the postmentum contains two plates (Fig. 84 A, 
rst). The mentum and submentum in some insects appear to be differ- 
entiations of a more primitive postmental plate; in others the mentum is 
evidently a secondary sclerotization in the membranous distal part of the 
postmentum. In adult Coleoptera the mentum is typically large and 
conspicuous (Fig. 158 C, Mt), but in the Orthoptera it is often reduced 
(Fig. 83 C) and is entirely absent in Mantidae and Acrididae. 



THE HEAD APPENDAGES 


151 


The proximal angles of the postmentum (or of the submentum) gen- 
erally preserve the primitive close association of the labial base with the 
posterior tentorial pits; but they may become far removed from the fora- 
men magnum if the postgenal regions of the cranium are elongate, or 
especially when a gular plate bridges the space between the postoccipital 
margins proximal to the labium (Figs. 67 C, 68, Gu). The base of the 
labium loses its association with the posterior tentorial pits only when 
mesal lobes of the hypostomal areas of the cranium are developed proxi- 
mal to its base (Fig. 164 C, Hst) or form a complete bridge between the 
labium and the foramen magnum (Fig. 65 B, C). 

Musculature of the Labium. — ^The muscles of the labium may be 
divided into four groups. Those of the first group are the muscles of the 
palpi and the terminal lobes; those of the second include several pairs of 
muscles inserted near the orifice of the duct of the labial glands ; those of 
the third group are the median muscles extending from the postmentum 
to the prementum; and those of the fourth are the extrinsic muscles of 
the labium arising on the tentorium and inserted on the prementum. 

The muscles of the palpi and the terminal lobes of the labium (Fig. 
81 A) correspond to muscles of the palpi, laciniae, and galeae of a pair 
of maxillae (B). Each labial palpus is provided with a levator and a 
depressor muscle arising in the prementum (Fig. 84 A, Iplp, dplp). The 
glossae and paraglossae have each a flexor muscle taking its origin in 
the prementum {fgl, fpgl), but the glossae have no muscles arising on the 
head wall corresponding to the cranial flexors of the maxillary laciniae 
(Fig. 78, flee). 

The labial muscles associated with the orifice of the labial, or salivary, 
glands have no homologues in the maxillae, and they are not always 
present in the labium. Generally there are two pairs of them, which arise 
in the prementum and converge to the labial wall of the salivary pocket 
formed at the junction of the hypopharynx with the prementum, into 
which opens the salivary duct (Fig. 84 A, B, 2s, 3s). These labial 
“salivary muscles” and the pair of opposing muscles from the hypo- 
pharynx (Is) will be more fully described in Chap. XII. 

The median muscles of the labium that extend from the postmentum 
to the prementum also have no homologues in the maxillae, and they are 
not always present in the labium. They arise on the postmentum, or on 
the submentum when there are two plates in the postlabial region, and are 
always inserted on the prementum (Fig. 84 A, B, rst). They are, there- 
fore, possibly sternostipital muscles, since there are never cardinostipital 
muscles in the maxillae. Usually these muscles are retractors of the 
prementum, but in some cases they serve to flex the prementum on the 
postmentum (Fig. 159 E). 



152 


PRINCIPLES OF INSECT MORPHOLOGY 


The extrinsic muscles of the labium comprise two pairs of muscles 
having their origins on the tentorium and their insertions on the pre- 
mentum (Fig. 84 A, B, ladlh, 2adlh). These muscles clearly correspond 
to the tentorial adductors of the maxillae, and in a morphological sense, 
therefore, they may be termed the labial adductors, though in their actual 
function it is probable that they produce various movements of the labium 



Fig. 84. — The labium and associated stnictures of Orthoptera. A, labial muscula- 
ture of Gryllus assimilis, dorsal (anterior) view. B, diagram of orthopteroid hypopharynx, 
salivarium, and labium, lateral view. C, salivarium and under surface of hypopharynx of 
Gryllus. D, salivarium and base of hypopharynx of a mantis, Paratenodera cinensis, 
dorsal view. E, diagrammatic section of salivarium of Paratenodera. 

besides that of adduction. One pair of the adductors is inserted ante- 
riorly, or dorsally, on the prementum {ladlh), the other posteriorly, or 
ventrally {2adlb). In some of the higher insects, as in the bees (Fig. 
163 C), the labial adductors may take their origin on the cranium, but 
this condition is evidently a secondary one resulting from a migration of 
the muscle bases from the tentorial arms to the adjacent cranial walls in 
order to give the fibers greater length and increased effectiveness. There 
are no head muscles inserted on the postmentum corresponding to either 
the cranial muscles or the tentorial adductors of the maxillary cardines, 
but the absence of these muscles in the labium is evidently consequent 
upon the usual immobility of the postmentum. 



THE HEAD APPENDAGES 


153 


Associated with the anterior (or dorsal) adductors of the labium 
there is usually present in generalized insects a pair of retractors of 
the hypopharynx (Fig. 84 B, rhphy). These muscles take their origins 
on the posterior bridge of the tentorium (Tnt) and are inserted laterally 
on the base of the hypopharynx (Hphy), where they are attached to the 
plates (w) of the latter, when these plates are present. 

8. GLANDS OF THE HEAD APPENDAGES 

Associated with the mouth parts of insects is a series of paired glands, 
which perhaps are coxal glands of the gnathal appendages. Some 
writers have attempted to correlate these glands with the nephridial 
glands of Crustacea, but the head glands of insects appear to be entirely 
of ectodermal origin. Glands occur also in connection with the antennae, 
but it is doubtful if they belong to the series of gnathal glands. 

Antermal Glands. — Glands connected with the antennae, so far as 
observed, are not of common occurrence in insects. In an ant, Myrmica 
rubra, however, Janet (1894, 1898) has described a group of one-celled 
anteimal glands, the ducts of which open separately in a small pit on the 
rim of the antennal socket. In the roach Periplaneta americana a small 
coiled tubular gland, mentioned by Bugnion (1921), opens at the base of 
each antenna. Perhaps a further search will show that antennal glands 
are more generally present in insects than the few records of their occur- 
rence would indicate. 

Mandibular Glands. — Glands associated with the mandibles are 
known to occur in Apterygota, Isoptera, Orthoptera, Coleoptera, Tri- 
choptera, larval Lepidoptera, and Hymenoptera. In the Apterygota, 
Willem (1900) reports the presence of head glands in Orchesella opening 
on each side of the hypopharynx near the bases of the mandibles, and 
Bruntz (1908) describes mandibular glands in Machilis maritima as 
“anterior cephalic glands,” each of which consists of a large racemose 
glandular mass with a principal lobe in the head and a smaller one in the 
thorax, the duct extending ventrally from the former to its opening in 
the preoral cavity at the base of the mandible. In the Orthoptera, Suslov 
(1912) found mandibular glands in Mantidae and Blattidae but dis- 
covered none in Gryllidae, Tettigoniidae, or Acrididae. The glands of 
Mantis religiosa, he says, consist each of a thick-waUed glandular sac and 
a thin-walled reservoir, the second opening to the exterior mesad of the 
posterior angle of the mandible. In the Hymenoptera also the glands of 
the mandibles are saclike with thick cellular walls. The mandibular 
glands attain their highest development in certain lepidopterous larvae, 
in which they have the form of long tubes extending often far back into 
the body cavity. The secretion of the mandibular glands probably has a 



164 


PRINCIPLES OF INSECT MORPHOLOGY 


“salivary” function in most cases; the size of the glands in some cater- 
pillars may be correlated with the transformation of the ordinary salivary 
glands (labial glands) into silk-forming organs. 

Maxillary Glands. — The presence of maxillary glands has been 
reported in Protura, Collembola, Heteroptera, the larvae of some Neur- 
optera and Trichoptera, and Hymenoptera; they occur also in some 
coleopterous larvae. The maxillary glands are usually small and 
inconspicuous, but in certain prionid larvae (Orthosoma) they consist of 
long convoluted tubes opening mesad of the maxillary bases and extending 
far back in the body cavity. 

Labial Glands. — The glands of the head appendages generally most 
highly developed in insects are those of the second maxillae, the ducts 
of which are united in a common median outlet tube (Fig. 84 B, SID) 
that opens typically in the pocket of the ventral wall of the head between 
the base of the free part of the labium and the base of the hypopharynx 
(Slv). These glands are commonly known as the “salivary glands”; 
but since their function is variable and has not been definitely determined 
in many cases, they are better termed the labial glands. 

In the embryo the labial glands originate as paired invaginations of the 
ectoderm just behind the bases of the rudiments of the second maxillary 
appendages. As development proceeds, the two orifices approach each 
other and unite medially on the venter of the second maxillary segment. 
At the same time, the appendages of this segment also come together and 
unite by their mesal edges. Meanwhile, however, the median aperture 
of the glands has moved forward, so that, when the labium is formed by 
the fusion of the second maxillary appendages, the outlet of the glands lies 
in the ventral wall of the head anterior to the base of the labium. Labial 
glands are present in all the principal orders of insects except 
Coleoptera. 

The size and shape of the labial glands are highly variable in different 
insects. Usually the glands lie in the thorax, but they may have a part 
in the head, and they often extend into the abdomen. Typically they 
are simple or convoluted tubes, but they may be branched or take on the 
form of dense racemose masses. A part of each lateral duct is sometimes 
enlarged to form a reservoir. The secretion of the labial glands generally 
has some function connected with feeding, though not necessarily that of 
a digestive fluid, for in blood-sucking insects it may have inflammatory 
and anticoagulatory properties. In lepidopterous and hymenopterous 
larvae the labial glands are silk-producing organs. Several writers have 
attributed an excretory function to the labial glands of Apterygota based 
on their reaction to ammoniacarmine and indigocarmine injected into 
the body, but as in the case of the so-called “ nephrocytes ” (see page 415) 
this test perhaps does not necessarily indicate an excretory function. 



TEE HEAD APPENDAGES 


155 


The salivary pocket, or salivarium (Fig. 84 B, Slv), at the base of the 
hypopharynx in generalized insects, into which opens the duct of the 
labial glands, is of much interest because of its various modifications in 
the higher order to form an organ for actively expelling the secretion of 
the glands. It becomes the “salivary syringe ’’ of Hymenoptera, Diptera, 
and Hemiptera, and the “silk press” of lepidopterous larvae. The 
salivarium in both its generalized and its specialized forms will be more 
particularly described in Chap. XII. 

GLOSSARY OF TERMS APPLIED TO THE HEAD APPENDAGES 

Antennae (Ant). — The appendicular organs of the procephalic region of the head 
innervated from the deutocerebral lobes of the brain; called first antennae, or anten- 
nules, in Crustacea; absent in Chelicerata. 

Cardo (Cd). — The proximal subdivision of a maxillary appendage. 

Chelicerae. — The first pair of appendages of adult Chelicerata, innervated from 
the tritocerebral ganglia of the brain; equivalent to the second antennae of Crustacea. 

First Maxillae (IMx). — The second pair of appendages of the gnathal region of 
the head; in insects called simply "the maxillae,” 

Flagellum (FI). — The part of the antenna distal to the pedicel, typically filamen- 
tous, but of various forms, usually subsegmented or multiarticulate. 

Galea (Go). — The outer endite lobe of a maxilla, provided with a muscle arising 
in the stipes. 

Glossae (Gl). — The two median ligular lobes of the labium, each provided with a 
muscle arising in the prementum. 

Gula (Gu). — ^A median ventral plate of the head of some insects, developed as a 
sclerotization of the gular region of the neck proximal to the posterior tentorial pits, 
continuous with the basal plate of the labium. 

Labial Glands (SlGl). — The usual “salivary glands” of insects, opening by 
a median duct between the base of the hypopharynx and the labium, or on the 
hypopharynx. 

Labial Suture (lbs). — The suture of the labium between the prementum and the 
postmentum, always distal to the mentum when the latter is present. 

Labiostipites (Lst). — The prementum, or that part of the labium formed by the 
stipites of the component labial (second maxillary) appendages. 

Labium (Lb). — The posterior median appendage of the insect head formed by the 
union of the second maxillae. 

Lacinia (Lc). — The inner endite lobe of a max-illa, provided with a muscle arising 
in the stipes, and often with a second muscle arising on the cranial wall. 

Ligula (Dig ). — The terminal lobes of the labium collectively, or a terminal part of 
the labium formed by the union of the lobes. 

Mandibles (Md). — The first pair of appendages of the gnathal region of the head in 
the Mandibulata; biting jawdike organs in their generalized form. 

Mandibular Glands. — A pair of glands often present in insects opening mesally 
at the bases of the mandibles. 

Maxillae (Mx). — The first and second maxillary appendages, or specifically in 
insects the first maxillae. 

Maxillary Glands. — Glands present in some insects opening mesally at the bases ef 
the maxillae. 



166 


PRINCIPLES OF INSECT MORPHOLOGY 


Maxillipeds. — The three pairs of appendages in Crustacea following the second 
maxillae; the first pair sometimes (Amphipoda) united to form a labiumlike structure 
attached to the head. 

Maxillulae. — The first maxillae of Crustacea. 

Mentum {Mt). — A distal plate of the postlabium between the prementum and the 
submentum. {Secondary submenial plate, Walker, 1931.) 

Palpifer {Plf). — A lobe of the maxillary stipes bearing the palpus. 

Palpiger {Pig). — A lobe of the stipital region of the labium, or prementum, bearing 
the palpus. 

Palpus {Pip). — The telopodite of a gnathal appendage. 

Paraglossae {Pgl). The lateral ligular lobes of the labium, each with a muscle 
arising in the prementum. 

Paragnatha {Pgn). — A pair of lobes of the gnathal region of Crustacea situated 
between the mandibles and the first maxillae. 

Pedicel {Pdc). — The second segment of the insect antenna, containing a special 
sense organ, the organ of Johnston. 

Pedipalps. — The second appendages of adult Chelicerata, corresponding to the 
mandibles of Mandibulata. 

Postantennal Appendages {Pnt). — The appendages of the tritocerebral somite; 
the chelicerae of Chelicerata, the second antennae of Crustacea, embryonic rudiments 
in some Hexapoda, absent in Myriapoda. 

Postmentum {Pmt). — The postlabium, or basal part of the labium proximal to the 
stipital region, or prementum; when sclerotized, containing either a single postmental 
plate, or a distal mental plate and a proximal submental plate. {Submentum, Walker, 
1931.) 

Preantennae {Prnt). — Theoretically a pair of primitive procephaUc appendages 
anterior to the antennae; possibly represented in Scolopendra and Dixippus by a pair 
of embryonic preantennal lobes; absent in all adult arthropods. 

Prelabium {Prlb). — The distal part of the labium, comprising the prementum, the 
ligula, and the palpi. {Eulabium.) 

Prementum {Prmi). — The stipital region of the labium, containing the muscles of 
the palpi and the ligular lobes, and giving insertion to the cranial muscles of the 
labium. {Mentum, Walker, 1931.) 

Salivary Glands {SlGl).- — See labial glands. 

Scape {Sep). — The basal segment or stalk of the insect antenna. 

Second Antennae. — The appendages of the tritocerebral somite of Crustacea. 
(See postantennal appendages.) 

Second Maxillae ifiMx). — The third pair of gnathal appendages; in insects united 
in the labium. 

Stipes {St). — The distal subdivision of a maxilla, bearing the endite lobes and the 
palpus, and containing the palpal and lobe muscles. (Plural stipites.) 

Subgalea {Sga). — A lobe or subdivision of the maxillary stipes bearing the galea. 

Submentum {Smt). — A proximal plate of the postlabium; when continuous with a 
gular plate the submentum lies distal to the posterior tentorial pits. {Primary 
submental plate. Walker, 1931.) 

Superlinguae {Slin). — A pair of ventral lobes of the insect head similar in some 
respects to the paragnatha of Crustacea, developed from the mandibular somite and 
united with the lingua in the hypopharynx of adult insects. 



CHAPTER Vni 


THE THORAX 


H- 




"Ll 


I 


MX 


Tho thorax of a winged insect is a highlj’ perfected bit of animal 
maclnnerj'. Insects are nnsuriiassed flj'ers, and few other creatures can 
make more effective and diversified uses of their legs. A thorax of the 
insect type is cxclusivelj’ a hexapod structure; it distinguishes the insects 
and proturans from all their relatives. Other 
members of the Arthropoda maj' have a body 
section called the "thorax,” but its .seg- 
ments do not correspond to those of the 
insect thorax, and its functions are bj’ no 
means as centralized or specialized. The 
thorax of the Ilexajioda consists of the three 
body segments following the gnathal seg- 
ments (Fig. 23 C, Th), which arc desig- 
nated, respectivelj’, the prothornx (Fig. So, 

Th\), the vicsothorax (Tliz), and tho incta- 
Ihornx (Thi). Each segment bears a p.air of 
legs (Ij), and the second and third segments 
carry the wings (11’;, ll’.n) in alate Pterygota. 

The thorax contains the muscles of the legs 
and wings, and the thoracic ganglia arc the 
chief centers of control for both sets of 
appendages. 

Between the thorax and the head is a 
narrowed, mostlj' membranous part of the 
trunk forming the neck, or cervix {Cvx). 

Though the neck is jirobably a comjiosite 
region formed from the labial and the pro- 
thoracic segments, it is more conveniently 
treated as a part of the thorax. 


Thi 


X 

— 



Ab- 


/ 


,Cer 


I'ki. S5. — Diiicram Bhowinp 
tlip rontiKuity of flip fprcnl .sclor- 
itp.s in the dorsum of tho winR- 
honriiiK rpRion of tho body to 
provent loiiRitudinnl movement 
of tho bnck pintes. 


1. EVOLUTION OF THE THORAX 

The thorax must have been evolved veiy carlj’’ in the ph 3 dogcnctic 
histoiy of the Hexajioda as a locomotor section of the bodj’^ through the 
specialization of its appendages for more active progression. In the 
Apterj-^gota as well as the Plciygota the thorax is distinctlj'' differentiated 

167 





158 


PRINCIPLES OF INSECT MORPHOLOGY 


from the abdomen in the structure of its segments, showing that the 
inception of the thorax as a body region long antedated the acquisition of 
wings. The thoracic region of the embryo (Fig. 23 C, Th) is well devel- 
oped as the leg-bearing part of the body at a stage when the gnathal 
segments (Gn) are yet distinct and show no evidence of their future union 
with the procephalic lobes (Prc). Concurrent with the specialization of 
the thoracic appendages as organs of locomotion, the abdominal append- 
ages were lost (Fig. 24 D), and the future gnathal appendages assumed 
functions accessory to feeding. When the gnathal segments were then 
finally combined with the protocephalon to become a part of the definitive 
head (E, ff), the hexapods appeared in their modern three-part form. 

The wings are acquisitions developed comparatively late in the evolu- 
tion of insects, though they are fully formed in the earliest known fossil 
forms. The wings are flat folds of the body wall extended from the lateral 
parts of the dorsum of the mesothorax and metathorax; they are thus in 
a sense homodynamous with laterotergal lobes that may occur on any of 
the body segments in both insects and other arthropods. That the 
wings belong to the dorsum is shown by the fact that the thoracic spiracles 
always lie below their bases. 

There is no evidence that true wings were ever present on the pro- 
thorax, but in many of the earlier fossil insects small lateral lobes project 
from the margins of the prothoracic tergum (Fig. 119, pnZ), suggesting 
that similar lobes on the mesothorax and metathorax were the precursors 
of the wings. The immediate ancestors of the flying insects, therefore, 
probably had three pairs of laterodorsal, or paranotal, flaps on the thorax, 
together forming broad extensions from the dorsum at the sides of the 
body. Evidently, then, in a second stage of their evolution, insects were 
enabled to depart from a strictly terrestrial or arboreal life by using their 
paranotal lobes as gliders on which they could launch themselves into 
the air from some elevation or sustain themselves after a preliminary 
leap from the ground. Later, a third stage was inaugurated with the 
transformation of the paranotal lobes of the mesothorax and the meta- 
thorax into movable organs of true flight. 

Each of the three stages in the evolution of modern insects from their 
generalized polypod ancestors has left its separate impression on the 
structure of the thorax. Hence, m a study of the thorax, we may observe 
three groups of characters, aside from the presence of the legs and wings, 
that distinguish the thorax from the other body regions. First, there are 
features common to the thorax of apterygote and pterygote insects that 
were probably evolved as direct adaptations to a more efficient use of the 
legs when the function of locomotion became localized in the thorax. 
Second, there are characters distinctive of the pterygote thorax not 
e^ddently related to the legs, but which are repeated in each segment. 



THE THORAX 


159 


and which, therefore, may be supposed to be correlated in their origin 
with the equal development of paranotal lobes on the prothorax, meso- 
thorax, and metathorax to form a glider apparatus. Third, there are 
characters peculiar to the mesothorax and metathorax of pterygote 
insects which undoubtedly have been acquired in connection with the 
evolution of the paranotal lobes of these segments into organs of flight. 

2. THE NECK 

The neck, or cervix, of insects is a narrowed membranous region of 
the trunk between the head and the thorax (Fig. 85, Cvx). It is usually 
short and mostly concealed within overlapping parts of the prothorax 
(Fig. 87, Cvx), but it is generally of greater length than it appears to be 
and is sometimes elongate and exposed (Fig. 99, Cvx). Some writers 
have regarded the neck as a reduced body segment (“microthorax”), 
but no conclusive evidence has been adduced in favor of this view; 
others have regarded it as a posterior part of the labial segment, and stiU 
others as an anterior part of the prothorax. The true morphology of the 
cervix is still obscure, but many structural features associated with the 
neck suggest that it includes parts of both the labial and the prothoracic 
segments (Fig. 87), and that it contains the primary intersegmental line 
between these segments (llsg). This view is in part substantiated by 
Smreczynski (1932), who says that in the embryonic development of 
Silpha obscura most of the second maxillary segment enters into the 
formation of the neck. 

Both the dorsal and the ventral series of longitudinal trunk muscles 
arise on the back of the head and extend through the neck. The prin- 
cipal dorsal muscles (Fig. 87, DMcl) are attached anteriorly on the 
postoccipital ridge of the cranium (PoR) and posteriorly on the antecosta, 
or phragma (IPh), of the mesothorax. The ventral muscles (VMcT) 
extend from the postoccipital ridge or the tentorial bridge (PT) to the 
apophyses of the prosternum. Neither the dorsal nor the ventral 
muscles, therefore, have connections in the prothorax corresponding to 
the usual antecostal attachments of intersegmental muscles. We have 
seen that the postoccipital ridge of the head most probably represents 
the intersegmental fold between the maxillary and labial segments. It 
is evident, therefore, that the intersegmental line between the labial 
segment and the prothorax lies somewhere in the membranous neck 
(llsg), and that the dorsal and ventral muscles of the neck and prothorax 
include the fibers normal to two segments ; that is, the muscles of the labial 
segment have become continuous with the muscles of the prothorax 
through the loss of then- attachments on the intersegmental fold between 
these two segments. It is quite obvious that some such structural modi- 
fication as this is necessary to give freedom of movement to the head; 



160 


PRINCIPLES OF INSECT MORPHOLOGY 


otherwise the activities of the head on the prothorax would be limited 
to the restricted movements of the ordinary intersegmental mechanism. 

On each side of the neck there is typically a pair of lateral cervical 
sclerites (Fig. 87, lev, 2cv). The two sclerites of each pair are hinged to 
each other, the first articulates anteriorly with the back of the head, the 
second posteriorly on the prothoracic episternum. The lateral neck 
plates not only link the head to the thorax, but the anterior ends of the 
first in each pair form two fulcral points on which the head can be tilted 
up and down by the dorsal and ventral muscles attached to it. There are 
muscles also inserted on the neck sclerites, some arising on the back of the 
head, others on the pronotum. The cervical plates and their muscles, 
therefore, when typically developed, constitute a protractor apparatus 
of the head, for the head is protruded when the angle between the plates 
is straightened by the contraction of the muscles. Either one or both of 
the lateral cervical sclerites may be absent, however, and when only one 
is present it is sometimes fused with the episternum (Fig. 99, cv). In 
some insects there are also dorsal, lateral, and ventral cervical sclerites, 
but these usually have no muscles connected with them. The general 
mechanism of the insect neck has been but little investigated; a review of 
the structure of the cervical sclerites may be found in several papers by 
Crampton (1917, 1926) and in one by Martin (1916). 

3. GENERAL STRUCTURE OF THE THORAX 

The thorax of an adult insect is in general easily recognized, since it is 
the section of the trunk bearing the legs, and the wings when wings are 
present. Ordinarily the thorax consists of the three body segments 
following the head, but in most of the Hymenoptera the fourth segment 
is so intimately associated with the third that it virtually becomes a 
thoracic rather than an abdominal segment. On the other hand, the 
thorax is often distinctly divided between its first two segments into a 
prothoracic part and a meso-metathoracic part. The second part, com- 
posed of the wing-bearing segments more or less closely united with each 
other, may be termed the pterothorax. 

In the present section we shall consider only those more fundamental 
features of the thoracic structure that presumably were developed before 
the paranotal lobes evolved into movable organs of flight; the structural 
modifications by which the pterothorax has been evolved into a mecha- 
nism of wing movement will be discussed separately. 

The Thoracic Terga. — ^The tergal plates of the thorax are usually 
modified in various ways, but the generalized structure is preserved 
in the mesothorax and metathorax of wingless insects. In the Apterygota 
and in nymphal and many larval Pterygota, the terga of these segments 
are simple back plates similar to those of the abdomen where a typical 



THE THORAX 


161 


secondary segmentation has been established (Fig. 37). Each plate 
comprises the primary segmental sclerotization of the dorsum and the 
preceding intersegmental sclerotization (Fig. 86 A). The definitive 
tergum, therefore, is crossed anteriorly by the line of the primary inter- 
segmental groove, which forms the antecostal suture (acs) externally and 
a submarginal antecosta (B, Ac) internally, and is thus divided into a 
narrow precostal acrotergite (atg) and a long postcostal area ending at the 
secondary intersegmental membrane following (Mb). The tergal ante- 
costae of generalized thoracic segments give attachment to the dorsal 
longitudinal muscles in the usual manner (Fig. 86 B, DMcl). In most 
winged insects, however, these muscles are greatly enlarged in the wing- 
bearing segments, and to accommodate them there are developed 



Fig. 86. — Diagrams illustrating intersegmental relations between the tergal plates of 

generalized segments. 


plate-like apodemal lobes from the antecostae of the mesotergum, the 
metatergum, and the first abdominal tergum. These antecostal apo- 
demes, which are usually paired but sometimes single, are known as the 
phragmata. Since there are typically three of them, the phragmata may 
be distinguished as the first phragma (Fig. 98, IPh), the second phragma 
(2Ph), and the third phragma (3Ph). 

The thoracic spiracles are generally situated on the sides of the 
segments, but the areas occupied by them must be supposed to belong to 
the dorsum, though they are beneath the wing bases in alate segments 
(Fig. 88, Sp). In Protura the spiracles are located in the lateral margins 
of the mesothoracic and metathoracic terga. Because of the dorsal 
extension of the thoracic pleura in insects, the thoracic spiracles are 
sometimes enclosed between the successive pleural plates. The first 
spiracle is mesothoracic, but it is often displaced anteriorly on the 
prothorax; the second is metathoracic and is also subject to an anterior 
migration. 

The Thoracic Pleura. — The insects resemble the chilopods in that 
there are associated with the functional leg bases one or more sclerites 



162 


PRINCIPLES OF INSECT MORPHOLOGY 


in the lateral walls of the leg-bearing segments. Evidence from ontogeny 
suggests that these so-called pleural sclerites belong to primitive subcoxal 
parts of the leg bases, and the fact that in both the chilopods and the 
insects some of the body muscles of the legs may be inserted on the pleural 



Fig. 87. — Diagram of intersegmental relations between the head and the prothorax, and 
between the prothorax and the mesothorax. 

areas gives a further reason for believing that the primary limb bases 
included not only the coxae but also the subcoxal areas of the body wall 
containing the pleural sclerites. In most other arthropods the 
coxopodites, or basal limb segments, are implanted in the pleural walls of 



Fig. 88. — Diagram of the theoretically primitive sclerotic elements of a thoracic seg- 
ment, in which the subcoxal part of the limb basis {Sex) includes two supracoxal sclerotic 
arches, the anapleurite {Apl) and coxopleurite {Cxpl), and an infracoxal arc, or alerno- 
pleurite {Spl). 

the body segments between the tergal and sternal plates, and thus not 
only does each coxopodite include the coxa of the leg but its base occupies 
the area of the subcoxa of chilopods and insects. The legs of many 
larval insects are borne on distinct subcoxal lobes of the thoracic body 


THE THORAX 


163 


segments, which contain the pleural sclerites in their dorsal walls (Figs. 
152 A, 153 A, Sex). 

The primitive subcoxal part of a thoracic leg probably formed a 
complete annulus proximal to the coxa (Fig. 88, Sex), which became 
flattened out in the pleural area of the body wall (P) to form a support for 
the rest of the limb. The entire subcoxal element in the body wall, 
therefore, includes not only the region of the pleural sclerites above the 
base of the coxa but also a ventral arc below the coxa. 

The subcoxal sclerotization becomes variously broken up into sclerites, 
but a study of the more primitive insects and the chilopods suggests that 
there were primarily three major sclerotic areas surrounding the base of 
the coxa, two concentrically placed above the coxa, and one below it. 
These are respectively the anapleurite (Fig. 88, Apl) situated dorsally, 
the eoxopleurite (Cxpl) closely associated with the upper rim of the coxa, 
and the sternopleurite (Spl) adjoining the sternum. The coxa is artic- 
ulated between the eoxopleurite and the sternopleurite (c, d). In most 
pterygote insects the two supracoxal arches unite to form the so-called 
pleuron, but they remain quite distinct in many Apterygota; the infra- 
coxal arc usually becomes a lateral element of the definitive sternum 
(Fig. 91 B, Ls). The pleuro- ventral line of a thoracic segment (Fig. 88, 
b-b), therefore, generally runs through the lateral part of the definitive 
sternal plate. 

The Apterygote Pleurites. — In the Apterygota the subcoxal sclerites of 
the thorax are small and variable and do not form definite pleural 
structures. A primitive condition in which each subcoxal area contains 
two distinct supracoxal sclerotic arches is well shown in some of the 
Protura and Collembola (Fig. 89, Apl, Cxpl) . If the coxa has a definite 
dorsal articulation in these forms it is with the eoxopleurite (C, c). The 
presence of a ventral articulation (d) probably means that the sterno- 
pleurite is contained in the definitive sternal plate. In Diplura and 
Thysanura the pleurites are variable and more or less degenerate, but 
in many cases there are distinct remnants of both the anapleural and the 
coxopleural arches. The ventral arc of the subcoxa more commonly 
preserves its independence from the sternum in the chilopods (Fig. 52 A, 
Spl) than it does in the insects. The thoracic pleurites of the Apterygota 
in many ways resemble the pleurites of the Chilopoda, and it is evident 
that in both groups the sclerites are in a degenerative state, since they 
have no very important function to perform. The highly developed 
pterygote pleuron, however, as we shall presently see, has apparently 
been derived from a more primitive pleural structure resembling that of 
the Apterygota and Chilopoda. 

The Pterygote Pleuron. — In the Pterygota the progressive evolution oi 
the supracoxal part of the subcoxa into an important skeletal part of the 



164 


PRINCIPLES OP INSECT MORPHOLOGY 


body segment, and the union of the infracoxal arc with the sternum have 
largely obscured the more primitive subcoxal structure exhibited by the 
Apterygota. It is only in the prothorax of Plecoptera that the pterygote 



Fia. 89. — Examples of the presence of two supracoxal arches in the thoracic subcoxal 
region, as shown in Fig. 88. A, mesothorax of Acerentomon doderoi. {From Berlese, 

1910.) B, Isotoma. C, mesothorax of Acerentulus barberi. {B, C from H. E. Evring, 

1928.) 

pleuron retains the apterygote condition in which the anapleurite and 
the coxopleurite are distinct sclerites (Fig. 90, ApZ, Cx'pl) . In all other 

cases these sclerites apparently are united in the single lateral plate 



Fig. 90. — Examples of the retention of a distinct anapleurite and coxopleurite in the 
prothorax of pterygote insects (Plecoptera). A, larva of Pteronarcys. B, larva of Perla, 
external view. C, same, internal view. 

supporting the coxa (Fig. 91 A, B). The prearticular part of the coxo- 
pleurite, however, generally remains as a partly or entirely free sclerite, 
the trochantin (Tn), the ventral extremity of which usually acquires an 
articulation with the anterior margin of the coxa (e). 




THE THORAX 


165 


The usual pterygote thoracic pleuron, formed by the union of the 
two supracoxal arches of the subcoxa, is typically a more or less con- 
tinuous sclerotic area in the lateral wall of the body segment, surrounding 
the base of the coxa dorsally, anteriorly, and posteriorly (Fig. 91, A, B). 
Above the coxa the pleuron is reinforced by a strong internal 'pleural 
ridge (Fig. 92 A, PIR) extending upward from the coxal articulation 
(CxP), which is formed by a linear inflection of the outer wall, known 
as the pleural suture (Fig. 91, PIS). In a wing-bearing segment both the 
pleural ridge and its suture are carried upward into the pleural wing 
process (B, WP), and in such cases the ridge braces the pleural wall 




Fia. 91. — Diagrams illustrating the apparent evolution of the pleural and sternopleural 
Bclerotization of a wing-bearing segment from the subcoxa. (Compare with Fig. 88.) 
The anapleurite and its ventral extensions become the episternum (B, Bps), the epimeron 
(Epm), the precoxale (Prcx), and the poslcoxale {Pcx); the anterior part of the coxopleurite 
forms the trochantin (Tn) ; the sternopleurite unites with the primitive sternum {Stn) and 
becomes a laterosternal element (Bs) of the definitive sternum. 


between the wing support and the coxal articulation. From each pleural 
ridge there projects inward and downward an apodemal arm, the pleural 
apophysis (Fig. 92 A, PI A), which is usually associated with a correspond- 
ing sternal apophysis (SA). 

The pleural suture divides the upper part of the pleuron into a pre- 
sutural episternum (Fig. 91 B, Eps) and a postsutural epimeron (Epm). 
The region of the pleuron extending downward from the episternum 
anterior to the coxa and the trochantin is the precoxal bridge, or precoxale 
(Prcx), generally united ventrally with the sternum; that behind the coxa, 
continuous from the epimeron and frequently united below with the 
sternum, is the postcoxal bridge, or postcoxale (Pcx). The precoxal and 
posteoxal sclerotizations may end ventrally in an infracoxal fold, evi- 
dently the infracoxal arc of the subcoxa (A) ; when they are united with 
the sternum it would appear probable that the ventral subcoxal arc has 
fused with the primary sternum (Stn) and forms a lateral part of the 


166 


PRINCIPLES OF INSECT MORPHOLOGY 


definitive sternum (B, Ls). The precoxal region sometimes forms a 
distinct sclerite separated from both the episternum and the sternum 
(Fig. 102 B, Prcx). The postcoxal sclerotization is seldom an inde- 
pendent sclerite, but it is often suppressed. 

The anterior remnant of the coxopleurite, known as the trochantin 
(Fig. 91 A, Tn), is best preserved in the more generalized pterygote 
insects. When well developed it bears at its anterior or ventral end the 
anterior trochantinal articulation of the coxa (B, e); and usually the 
tergal promoter muscle of the leg is inserted on it. In the higher insects 
the trochantin becomes reduced or obliterated, or it may be united with 
the lower margin of the episternum in such a way that its limits are often 
difficult to determine. 



Fig. 92. — Diagrammatic cross section of thoracic segments illustrating the evolution 
of the furca (B, Fu) from the sternal apophyses (A, SA) and the inflected median part of 
the sternum (<S). 


The thoracic pleuron of the Pterygota is thus seen to differ consist- 
ently from the variable and weakly developed pleural parts of Apterygota 
in that it forms, in the adult stage of the insect, a definite and elaborate 
structure in the lateral wall of the body segment between the coxa and 
the tergum. Moreover, the basic features of the pterygote pleuron are 
the same in the wingless prothorax as in the alate mesothorax and meta- 
thorax. We cannot, therefore, attribute the characteristic structure of 
the pterygote pleuron to the development of the wings. On the other 
hand, we may suppose that the thoracic pleura of winged insects acquired 
their fundamental characters in correlation with the development of 
paranotal lobes on all the thoracic segments in the preflying glider stage 
of insect evolution. 

The Thoracic Sterna. — The degree of sclerotization in the ventral 
walls of the body segments varies much in different arthropods; in some 
the venter is entirely membranous, in others it is occupied by definite 
sternal plates. Though a typical sternum (Fig. 36 B, Stn) includes the 
preceding intersegmental fold (Ac) on which the principal ventral body 


THE THORAX 


167 


muscles are attached, the venter may be occupied bj’’ a series of alternat- 
ing segmental and intersegmental sclerites, as in some of the chilopods 
(Fig. D8). 

The sternal plates of the thorax in adult insects differ generall}^ in 
three respects from those of the abdomen: first, in the independence 
of the primary segmental and intersegmental sclerotizations, or in the 
opposite relation of the second to the first when the two are united; 
second, in the reversed overlapping of the plates at the secondar}’’ inter- 
segmental lines; and, third, in a transposition of the attachments of most 
of the ventral muscles from the intersegmental to the segmental sclerites. 

Basic Structure of the Thoracic Sterna. — In the thorax the interseg- 
mental sclerites of the venter between the prothorax and the mcsothorax 
and between the mesothorax and the metathorax are never united with 
the segmental plates following; either they remain as free intersternites 
(Fig. 37, list, 2Ist) or they unite with the segmental sterna preceding. 
The primary intersegmental sclerotization behind the metasternum is 
generally lost or is united vdth the abdominal sternum following. 

The segmental plate of the venter of a thoracic segment ma}' be 
designated the eusternum (Figs. 93 A, B, Stn, 96, ES). The intersternites 
of the thorax (Fig. 93 A, 1st) are commonly termed spinasterna (Figs. 
93 B, 96, Ss), because each usually bears a median apodemal process 
called the spina (Fig. 87, Spn). 

Since both the first and the second spinasterna may be free inter- 
sternal sclerites, or the second, or also the first, may unite with the 
eusternum preceding, we usually encounter one of the following three 
series of sclerites in a study of the thoracic sterna, except when the 
eusternum itself is secondarily subdivided: (1) eusternum of prothorax, 
first spinasternum, eusternum of mesathorax, second spinasternum, 
eusternum of metathorax; (2) eusternum of prothorax, first spinasternum, 
composite mesosternum, eusternum of metathorax; (3) composite 
prosternum, composite mesosternum, eusternum of metathorax. A 
fourth condition may arise when the sternal plates of the wing-bearing 
segments are all united in a large pterothoracic plastron. It should be 
observed that the metasternum never hasa spinasternite, because the third 
intersternite either is suppressed or becomes the acrosternite of the first 
abdominal sternum. 

The eusternum of a thoracic segment, as we have obseiwed, usually 
comprises the primary sternal plate and the subcoxal sternoplouritos, 
the latter constituting the laterosterniies, or picurosternites, of the defini- 
tive sternum (Fig. 93 D, Ls). In some insects the thoracie sterna an* 
bordered bj’’ distinct subcoxal folds continuous with the pleura before and 
behind the coxae (Fig. 95 A); in others the limits of the laterosterniies 
are suggested by submarginal sutures; but in general the prc.scncc of 



168 


PRINCIPLES OF INSECT MORPHOLOGY 


lateral subcoxal derivatives is not evident in the definitive sternal plates 
of adult insects (Fig. 96, ES) and is only to be inferred from the continuity 
of the sternum with the precoxal and postcoxal bridges of the pleuron 
or from the presence of a ventral articulation of the coxa with the sternum, 
though the latter probably is secondary in some cases. 

The Reversed Overlapping of the Thoracic Sterna. — The sternal plates of 
the thorax characteristically overlap each other anteriorly (Fig. 37) and 
thus present a relation just the opposite from that prevailing in the 
dorsum, and in the venter of the abdomen. This reversed overlapping of 
the thoracic sterna is particularly striking in some of the Apterygota 
and in the more generalized Pterygota, but it is evident wherever the 
successive sternal plates are not united with each other. It is apparently 
correlated with the reversed relations of the intersegmental spinasterna 
to the eusternal plates, and, while the reason for this peculiarly thoracic 
modification is not clear, it must have some important significance in the 
mechanism of the thorax not connected with the wings. As a conse- 
quence, the sternum of the metathorax (Fig. 37, Stns) stands usually as a 
dividing plate overlapping in both directions between the mesothorax 
and the abdomen, though sometimes it also is overlapped by the first 
abdominal sternum. 

The Transposition of the Ventral Thoracic Muscles. — Associated with 
the reversed overlapping of the thoracic sterna, but not necessarily 
correlated with it, there occurs a partial or complete transfer of the 
attachments of the longitudinal sternal muscles from the intersternites 
to the preceding segmental parts of the definitive sterna. Theoretically 
we must assume that both the dorsal and the ventral muscles were 
originally attached on the lines of the primary intersegmental folds, 
which are preserved as the antecostae of the terga and of the abdominal 
sterna, but which are reduced in the sternal region of the thorax to the 
small, median, spinal processes of the spinasternites (Fig. 87, Spn). In 
adult pterygote insects the principal groups of ventral muscle fibers in 
the thorax extend between paired apophyses arising on the eusternal 
plates (/8Ai, 5 A 2 ), though a few fibers usually preserve the original 
connections with the spinae (Spn). In most holometabolous larva, on 
the other hand, the principal ventral muscles throughout the length of 
the body are regularly attached on the intersegmental folds. The adult 
condition, then, is evidently a secondary one. 

The Thoracic Sterna of Apterygota. — The sternal sclerotizations of the 
thorax in the Apterygota are variously developed and show no progressive 
evolution within the group. In the Protura and in Japyx the principal 
sternal plate in each segment bears an internal median ridge which may 
be forked anteriorly, with the arms extending to the ventral articulations 
of the coxae. Endosternal structures are absent in Lepismatidae, where 



THE THORAX 


169 


each sternum is produced posteriorly into a large, scalc-likc lobe. In 
Machilidae the thoracic sterna are weakly developed areas of sclerotiza- 
(ion between the leg bases, seiiarated Iiy anpile intersegmental spaces. 
From each intersternal area a pair of delicate apodernal arms projects 
inward from a common median ba.‘~-e, forming thus a series of furca-likc 
structures. The intersegmental position of their bases, however, allies 
these apodernal structures with the median jirncesses of the spinasternites 
in the Pterygota rather than with the true sternal apophyses. 

T/ir Tlinractc Strrnn of Plrrijpofn. — 'I'lie sterna of pterygote insects 
arc characterized by the jiosse.'^sion of ])aired apophyses arising from 
the custernal plates. The stenidl apophi/srs- (I"ig. 92 A, <S.l) are often 



Flo. 03. — Ilincr.'itii*! tlip evolution of n KonrrMlir.eil (iefiiiilivc (hornoic sternum 

(D) by union of tlie jirimitive sterinim (.-V. Slu) witli tlic iufr.^poxnI nres of tlio sul)COxno 
(B, C, .Sex), nnd witli tlie followiui; interstornilo (.\. /rf), tvliicli Ijoroines the spimistornum 
(B, C, D, .SV). Tlie definitive eusternuin is finnlly divided Iiy tlie Hteriincostnl suturo 
(C, D, /;) into basinItTiium (Ilf) nnd rtrrnrllum (.SI), nnd mny linvc n nnrrow jtrcstcnuim 
(Prs) get off by nn nntorior subninrKinnl suture 07. 

called the furcal armif, bccatt.se in the higher pterygote orders the two 
apophyses in each segment are sujtported on a median inflection of the 
sternum and thus become the divergent prongs of a forked cndoskcletal 
structure known as the fiirca (B, I'u). I’he outer ends of the sternal 
apophj'scs are closclj' a.ssoci.ated with the inner ends of the pleural arms 
{PI A) of the same segmenti, the two pairs of processes being usuallj’’ con- 
nected by short muscle fibers, or in some cases fused with each other. 
When the pleural and sternal proce.‘;ses arc united on each side of the 
segment, they form a buttrc-sslikc arch across the co.val cavity from the 
sternum to the pleuron. 

The sternal apophyses support the principal longitudinal ventral 
muscles of the thorax, and they give attachment to some of the ventral 
muscles of the legs. Externally their roots are marked by a pair of 



170 


PRINCIPLES OF INSECT MORPHOLOGY 


pits in the sternum between the coxae (Fig. 93 B, sa, sa) . The primitive 
position of the apophyses is doubtful; according to Weber (1928, 1928a), 
the processes are invaginations between the lateral edges of the primary 
sternum and the subcoxal laterosternites, but the location of the external 
pits of the apophyses does not always conform with this view. 

In the more generalized Pterygota, the bases of the sternal apophyses 
are often connected by an internal transverse ridge, the sternacosta (Fig. 
92 A, k), the line of which appears externally as a siernacostal suture 
through the apophyseal pits (Fig. 93 C, D, k). The sternal suture 
divides the surface of the eusternum into a presutural area, or basisternum 



Fig. 94. — An example of desclerotization in the venter of the thorax. A, thoracic 
sterna of Blatta orientalis. B, diagram of the typical sternal sclerotization of a thoracic 
segment. C, result of desclerotization in the prothorax of Blatta. D, result of extreme 
desclerotization as in the mesothorax or metathorax of Blatta. 

{Bs), and a postsutural area, the sternellum {ST), or “furcasternum.” 
When the anterior part of the eusternum is reinforced by an internal 
submarginal ridge, there is formed externally a presternal suture (D, j), 
which sets off a narrow marginal area of the sternum, called the pre- 
sternum {Prs). Muscles are never attached on the presternal ridge, and 
the latter should not be mistaken for a true antecosta; the sternal ante- 
costae of the thorax are represented by the spinae of the spinasternites. 

In its surface structure the thoracic sternum departs in many ways 
from the simple divisional pattern shown at D of Fig. 93. The sterna- 
costal suture is subject to variations in form, being often produced 
forward and variously branched, giving rise to an endoskeletal structure 
of diversified form. In some cases also convergent ridges extend pos- 
teriorly from the bases of the apophyses and may unite in a median 




THE THORAX 


171 


ridge, thus forming a Y-shaped endosternal ridge (Y-Leiste of Weber, 
1933), the external sutures of which cut the sternellum into median and 
lateral areas. Or, again, a confusing condition may arise from a partial 
desclerotization of the sternal plates, as in the mesothorax and meta- 
thorax of Blattidae (Fig. 94 A), where the eusternum in each of these 
segments is divided into one or two anterior basisternal sclerites (Bs) 
and a posterior sternellar sclerite (SI) supporting the long apophyses. 
The prosternum of the roach (C), however, retains more nearly the 
generalized sternal structure (B), and it is not difficult to see how the 
sclerite pattern of the mesosternum (D) or of the metasternum has been 
derived from the former (B) by a loss of sclerotic continuity. 



Fig. 95. — Examples of highly modified sternal sclerotization in the thorax. A, 
pterothorax of Magicicada septendecim, ventral view. B, thorax of CalHphora, ventral 
view. C, mesothoracio furca of CcUliphora, dorsal view. 

In the higher pterygote orders tne sternal apophyses are carried 
inward upon a median inflection of the sternum to form the Y-shaped 
endosternal apodeme known as the furca (Fig. 92 B, Fu). The stalk of 
the furca may arise from a deflnite pit marking the diWsion between the 
basisternal and sternellar regions of the sternum (Fig. 95 A, Bs 2 , Sh), 
or the part of the sternum bearing the furca may become detached as a 
distinct furcasternum (B, Fss). In many insects, however, the base of 
the furca is extended forward as a long median ridge through the whole 
length of the sternum (C), the site of which is marked externally by a 
median sternal groove (B, fu 2 ). In this case there is no distinction 
between basisternum and sternellum or furcasternum, and it is impossible 
to say how much of the true sternum has been inflected to form the furcal 
base. 

In a study of the thoracic sterna of the higher insects it seems more 
advisable to accept the facts as they are, unless identities with the sternal 
regions of more generalized insects can be traced through a series of 


172 


PRINCIPLES OF INSECT MORPHOLOGY 


families. In the pterothorax of the higher Diptera, for example (Fig. 
95 B), the more primitive sutures of the sternal as well as the pleural 
areas have become almost wholly obliterated, and secondary grooves 
appear which divide the skeletal surface into parts that have little 
relation to those in more generalized orders. The large ventral plate 
of the mesothorax of CalUphora (Fig. 95 B) is evidently composed of the 
sternum, the precoxal bridges, and parts of the episterna; the bridge (x) 
separating the middle and hind coxae must include the postcoxalia of 
the mesothorax, the precoxalia of the metathorax, and the metathoracic 
basisternum. The small sclerite (Fsz) between the hind coxae is a 
detached furcasternum, though the furca is supported also on the plate 
(x) before it. The prosternum, on the other hand, retains the more 
generalized structure in that the bases of the sternal apophyses (saO 
separate a long basisternal sclerite (Bsi) from a small sternellar region 
{Sh), which is united with the mesosternum. A presternal sclerite (Prsi) 
is here entirely cut off from the basisternum. 

4. THE PROTHORAX 

The prothorax differs consistently from the other body segments 
in that its tergum and sternum always lack the antecostal and precostal 
elements of typical segmental plates, these parts apparently having been 
lost by membranization in the neck. The prothoracic tergum is a plate 
of the primary segmental region only (Fig. 87, Ti). It never bears a 
phragma, since the fii-st phragma (IPh) is never detached from the meso- 
tergum, and the acrotergite of the mesotergum is not sufficiently enlarged 
to constitute a postnotum of the prothorax. The principal dorsal muscles 
of the prothorax (DMcl) extend through the segment from the postoccipi- 
tal ridge of the head (Pof?) to the antecosta, or phragma {IPh), of the 
mesotergum, but shorter muscles may connect the head with the proter- 
gum or the latter with the mesotergum. The size and form of the 
prothoracic tergum are highly variable. In some insects, as in Orthop- 
tera, Hemiptera, and Coleoptera, the protergum may be a large plate, 
sometimes greatly expanded; but since, in general, the back plate of the 
prothorax has little specific function aside from giving attachment to the 
dorsal muscles of the legs, it frequently assumes strange and fantastic 
shapes, or, on the other hand, it is reduced in size and may be but a 
narrow band between the head and the mesothorax. In some of the 
Hymenoptera the protergum is so intimately associated with the meso- 
tergum that the isolated pleurosternal parts of its segment form a free 
suspensorium for the first pair of legs. When the protergum is well 
developed its surface may be marked by sutures, which form ridges 
on the inner surface of the plate; but the resulting “divisions” of the 
protergum have no relation to those characteristic of the Tving-bearing 



THE THORAX 173 

terga. The internal ridges are usually found to have an intimate relation 
to muscle attachments. 

The prothoracic sternum has the same fundamental structure as the 
sterna of the pterothorax but is commonly more generalized than the 
latter. The eusternum bears a pair of apodemal apophyses (Fig. 87, 
jSAi), and the spinasternum (ISs), which may be widely separated from 
the eusternum or fused with it, bears a median spina. The true inter- 
segmental line between the prothorax and the mesothorax {2Isg) runs 
through the spinasternite ventrally and the base of the first phragma 
dorsally, but usually a wide membranous area constitutes the functional 
intersegmental conjunctiva. 

The pleuron of the adult prothorax resembles the pleural sclerotiza- 
tion of the pterothorax of nymphal and larval insects in that it lacks 
the alar development characteristic of the adult pterothoracic pleuron. 
In its general features it has the same type of structure as the pleura 
of the wing-bearing segments, and only in the Plecoptera (Fig. 90) does 
it show any suggestion of the more primitive structure of the apterygote 
pleuron. The episternum and epimeron are always well separated by a 
pleural suture and ridge, though the epimeron is often much reduced or 
fused with the margin of the tergum. In some Orthoptera the episternum 
is largely concealed within a lateral fold of the protergum, but, since it 
gives origin to the abductor muscle of the coxa, it is seldom reduced in 
proportion to the reduction of the epimeron. The lateral sclerites of 
the neck usually articulate with the prothoracic episterna, but they 
may be fused with the latter to form a pair of arms projecting from the 
propleura to support the head. Precoxal and postcoxal extensions of 
the supracoxal pleurites, of which one or both may be continuous vfith 
the sternum, are commonly present in the prothorax as in the ptero- 
thorax. The entire structure of the prothoracic pleuron suggests an 
evolution homodynamous with that of the pleura of the wing-bearing 
segments to a point where the latter became specialized as parts of the 
wing mechanism. 


6. THE PTEROTHORAX 

The wing-bearing segments differ structurally from the prothorax 
only in details that are clearly adaptations to the function of movement 
in the wings. The modifications affect chiefly the terga, in a lesser 
degree the pleura, and least the sterna. 

General Structure of the Wing-bearing Segments. — ^The typical 
structure of a wing-bearing segment is shown diagrammatically in Fig. 
96. The dorsum of the segment may be occupied entirely by a single 
tergal plate {AN), which bears the wings; but usually the segment in 
which the wings are better developed contains also a second, posterior 



174 


PRINCIPLES OF INSECT MORPHOLOGY 


plate {PN), which carries a phragma {Pph). Since entomologists 
generally prefer the term notum for the tergal plates of the thorax, we 
may designate the wing-bearing plate in the dorsum of a pterothoracic 
segment the alinotum (AN), and the phragma-bearing plate the phrag- 

manotum, or postnotum (PN) . The 
alinotum is often supported on the 
pleura by prealar arms (Pra) 
extending laterally or downward 



episterna; the postnotum is gener- 
ally firmly braced upon the pleura 
by lateral postalar extensions (Pa) 
united with the epimera. 

Since the phragmata are inflec- 


(Fig. 96, acs), the phragma- 
bearing postnotal plates of the 
dorsum are in every way compar- 
able with the spinasterna of the 

Fio. 96.-Diagram of the typical scler- The true interseg- 

ites of a wing-bearing thoracic segment, and mental lines 01 the thorax \Isg) 

their subdivisions, lateral ^ew. acs, dorsally through the bases of 

antecostal suture; AN, alinotum; ANP, i 

anterior notal v-ing process; Aph, anterior the phragmata, and ventrally 

phragma, prephragma; a<£, acroterdte; Sa, through the bases of the spinae. 
basalare; ^ 5 , basisternum; Ca;C, coxal cavity; ® , r v 

CxP, pleural coxal process; Epm, epimeron; The phragma-bearing plates of the 

Eps, episternum; ES eusternum; Isg dorSUm, however, differ from the 
primary intersegmental hne; k, sternacostal . ' _ r i 

suture; Mb, conjunctiva, secondary inter- Spina-bearing plates of the venter 

segmental membrane; Pa postalare; P(^, that they may be more closely 
postcoxale; pla, root of pleural apophysis; ^ , • r j.u 

PIS, pleural suture; PN, postnotum, phrag- aSSOCiatod OT United with the 

manotum; PiVP, posterior notal v-ing proc- segmental plate either before or 
ess; I'pri, posterior phragma, postphragma; , ^ . 

Pra, prealare; Prcx, precoxale; Prs, prester- behind them. ThuS the segment 

num; Prsc, prescutum; Rd, posterior fold carrying the principal pair of wings 
or reduplication of alinotum; >Sa, subalare; o t' t' ir r j 

Sd, scuteiium; Set, scutum; SI, sterneiium; may have a phragma at each end 

Ss spinasternum; Tn, trochantin; TFP, jtg tergal region. The anterior 
pleural wing process. & 

phragma in this case may be dis- 
tinguished as a prephragma (Aph), and the posterior one as a postphragma 


(Pph). 

The wings are flat folds of the body wall extending laterally from the 
edges of the alinotal plates (Fig. 85, W 2 , Ws), their upper membranes 
being continuous vdth the dorsal integument, their ventral membranes 
reflected into the lateral walls of the segments. The posterior border 
of each wing is continuous with the posterior marginal fold of the alinotum 



THE THORAX 


175 


(Fig. 96, Hd), but anteriorly the wing base ends behind the prealar arm 
of the notum. 

The pleural sclerotization of a wing-bearing segment is usually well 
developed and is almost always divided by a pleural suture (Fig. 96, FIS) 
into an episternum (Eps) and epimeron (Epm) . At the upper end of the 
suture the dorsal margin of the pleuron is produced into a pleural wing 
process {WP), which serves as a fulcrum for the movement of the wing. 
Before and behind the wing process in the upper membranous parts 
of the pleural wall, there are situated two or more epipleurites {Ba, Set), 
usually small plates upon which are inserted important muscles of the 
wings. Ventrally the pleuron is generally supported on the sternum 
by the precoxal and postcoxal bridges {Prex, Pcx). The trochantin 
(Tn) is variable and is usually suppressed in the higher orders. 

The sterna of the pterothoracic segments have no special features 
to distinguish them from the prothoracic sternum, except for the size 
of the basisternal regions, which are usually enlarged to accommodate 
the ventral ends of the tergosternal wing muscles, representatives of 
which are absent in the prothorax. 

The Tergal Plates of the Pterothorax. — The terga of the wing- 
bearing segments not only support the wings but are themselves important 
elements in the mechanism for moving the wings, since each acts as an 
intermediary between the indirect wing muscles of its segment and the 
bases of the wings. The tergum plays its part in the production of 
wing movement by responding to the contraction of the longitudinal 
dorsal muscles with an upward curvature between its two ends, and by 
a reverse action to the downward pull of the antagonistic tergosternal 
muscles. The depression of the tergum causes the upstroke of the 
wings; the dorsal flexure assists in the downstroke. It is evident that 
the effect of the dorsal muscles on the pterothoracic terga must depend 
on a close connection between the mesotergum and the metatergum 
and between the metatergum and the first abdominal tergum; otherwise 
there would be much lost motion, since, with the usual intersegmental 
relations, the contraction of the dorsal muscles simply pulls the tergal 
plates together. 

To accommodate them to their parts in the wing mechanism, the 
alate terga have been modified in three principal ways. In the first place, 
in order that the tergal plates may respond by changes in their dorsal 
curvature to the action of the longitudinal muscles attached on them, the 
intersegmental membranes have been reduced or eliminated, usually 
by a redistribution of the intersegmental sclerotization between the 
mesotergum and metatergum and between the metatergum and the first 
abdominal tergum, which has given rise to the so-called postnotal plates. 
In the second place, the wing-bearing plates have been strengthened, in 



176 


PRINCIPLES OF INSECT MORPHOLOGY 


order to withstand the strain of work imposed upon them, by the develop- 
ment of various ridges on their inner surfaces. These ridges are formed 
by linear inflections, or “sutures,” of the outer surfaces. Consequently, 
a wing-bearing tergum is subdivided by its ridges and their sutures into 
several distinct areas characteristic of the terga of the pterothorax, hut 
having no morphological counterparts in the tergal plates of other 
segments. Finally, since the wings are movable by definite articulations 



Fig. 97, — Evolution of the postnotal plates of the mesothorax and metathorax from 
the acrotergites of the segments following, and the development of the phragmata and 
dorsal muscles. A, generalized condition. B, postnotal plates formed as extensions 
of the acrotergites. C, postnotal plates with phragmata cut off by secondary membranes 
{Mb') from tergal plates following, D, section of Dissosteira, showing postnotum developed 
only in metathorax. E, section of Calliphora, showing postnotum in mesothorax, and 
metathoracic tergum almost obliterated on median line. 

on the supporting back plates, the lateral margins of the mesotergum 
and the metatergum present structural features precisely adapted to the 
hinging of the wing bases on the dorsum. 

Redistribution of the Intertergal Sclerotization in the Wing-hearing 
Segments. — The mesothoracic and metathoracic terga of apterygote 
insects and of nymphal and larval forms of pterygote insects having a 
well-developed thoracic sclerotization are the same as the abdominal 
terga in that each tergal plate comprises the segmental and preceding 
intersegmental sclerotization and is crossed anteriorly by a submarginal 
antecostal suture (Fig. 86). It is evident, therefore, that the immediate 
ancestors of the winged insects had a typical secondary segmentation 



THE THORAX 


177 


throughout the dorsum of the thorax and abdomen (Fig. 97 A). This 
structure is retained in the mesothoracic and metathoracic terga of some 
adult ninged insects, such as the Isoptera, in which the dorsal thoracic 
muscles are small and weak and probably haA^e little to do with moving 
the wings. The pterothoracic and first abdominal terga of the Isoptera, 
however, are closely attached to each other. A similar structure occurs 
in the Blattidae, in which also the small dorsal muscles are relatively 
unimportant elements in the wing mechanism by comparison with those 
of most insects. The successive terga in the wing region of Blattidae 
are connected by lateral expansions of the acrotergites of the metatergum 
and the first abdominal tergum. In both Isoptera and Blattidae there 
may be small phragmatal lobes on the antecostae of the mesotergum 
and metatergum. 

With the majority of winged insects the dorsal muscles of the ptero- 
thoracic segments are greatly enlarged, and their ends are attached 
on well-der-eloped phragmata depending from the antecostae of the 
mesotergum, tlie metatergum, and the first abdominal tergum (Fig. 97 B). 
The phragmata, as we have seen, are intersegmental inflections, and the 
external groo\"es, or antecostal sutures (acs), through their bases mark 
the primary intersegmental lines. The acrotergite (aig) of the meso- 
tergum (T 2 ) retains the usual form of a narrow flange before the antecostal 
suture (A, B); but the acrotergite of the metatergum and the acrotergite 
of the first abdominal tergum (B) are each enlarged and extended forward 
to the posterior margin of the tergum preceding in each case. In this 
way the dorsal intersegmental membranes (A, Mb) are practically 
eliminated between the mesotergum and the metatergum, and between 
the latter and the first abdominal tergum, and are replaced by the 
expanded acrotergites, which become postnatal plates of the mesothorax 
and the metathorax, respectively (B, PN 2 , PNs). 

The obliteration of the dorsal intersegmental membranes by the 
development of acrotergal postnotal plates produces a continuous sclero- 
tization in the dorsum of the pterothorax (Fig. 85) from the base of the 
first phragma (Fig. 97 B, IPh) to that of the third (3Ph). The con- 
tractile force of the dorsal muscles is thus prevented from pulling the 
successive terga together and is therefore expended against the tergal 
plates themselves, which may now respond by an upward curvature, 
producing a depression of the wings on the pleural fulcra. The elimina- 
tion of the secondary intersegmental membranes by the enlargement 
of the acrotergites virtually restores the pterothoracic dorsum to a 
condition of primary segmentation, since the functional segmental limits 
are now marked by the primarily intersegmental phragmata. The 
postnotal plates are thus seen to belong morphologically each to the 
segment of the tergal plate preceding it. 



178 


PRINCIPLES OF INSECT MORPHOLOGY 


If the hind wings are the principal organs of flight, as in Orthoptera 
and Coleoptera, a postnotum is developed in the metathorax only 
(Fig. 97 D, PNz). With most of the higher insects, however, in which 
the fore wings are large and the hind wings small, there is usually present 
a postnotal plate in each of the alate segments, though the second is 
generally reduced in size, as is also the alinotum of the same segment. 
An extreme adaptation to the two-winged condition occurs in the higher 
Diptera (E), in which the metatergum is a scarcely perceptible rudiment 
{Tz), and practically the entire dorsum of the thorax is formed of the 

alinotum and postnotum of the meso- 
thorax, between which, or their respective 
phragmata (IPh, 2Ph), extend the great 
dorsal muscles of the mesothorax. 

In many of the higher insects the base 
of each phragma becomes separated from 
the tergum behind it by a transverse line of 
membranization (Fig. 97 C, Mb'), and in 
such cases the postnotum (Fig. 98, PNz, 
PNz) includes not only the acrotergite (atg) 
but also the narrow posterior lip of the 
phragmatal inflection behind the antecostal 
suture. With insects having this type of 
structure, the functional conjunctivae on 
the dorsum are the membranes (Mb') 
behind the bases of the phragmata, and the 
tergal plates of the metathorax and first 
abdominal segments are incomplete by the 
lack of their usual anterior phragma- 
bearing parts. Generally the posterior lip 
of a detached postnotum is very narrow or 
scarcely apparent, but in some cases it is large, as in the metathorax of 
Panorpa (Fig. 99, PNz), where it includes the major part of the first 
abdominal tergum (IT). 

Each phragma consists typically of a pair of thin, plate-like apodemal 
lobes (Fig. 98, Ph) separated by a median notch giving passage to the 
dorsal blood vessel; but in some cases the two lobes are united in a single 
broad plate, and the blood vessel then dips beneath the latter. In 
certain H 5 nnenoptera the median part of the second phragma is mem- 
branous, giving the phragma the appearance of being connected with the 
tergum only by its lateral angles. 

The Sutures, Ridges, and Surface Areas of the Alinotum. — The sur- 
faces of the alinotal plates are greatly diversified in different insects 
by topograpliical irregularities and by sutures. The so-called sutures 



Fig. 98. — Diagrams shovring 
the derivation of the phragma- 
bearing postnotal plate (PN), 
associated with each wing-bearing 
alinotal plate (AN), from the 
tergum following in each case. 


THE THORAX 


179 


are mostly the external grooves of internal ridges, which are the important 
mechanical features of the notum, but the sutures are the characters 
more generally used in descriptive works. The principal ahnotal sutures 
and the areas they define may be described as follows: 

The antecostal suture is the groove through the base of a phragma 
which marks the line of the antecosta (Fig. 100 A, acs). The acrotergite 
before the antecostal suture is usually a very narrow anterior lip of the 
alinotum (citg), except when it is enlarged to form the postnotal plate 
of the preceding segment (Figs. 96, 98). 

The scutoscutellar, or V-shaped, suture (Fig. 100 A, vs) hes in the 
posterior part of the alinotum with its apex directed forward; it divides 
the notum into an anterior scutum (Set) and a posterior scutellum (ScT). 
Internally this suture forms usually a strong V-shaped ridge (B, VR), 



which not only strengthens the notum but probably, in its typical 
form, serves also as a gradient device to bring the peak of the upward 
flexure of the notum during flight on a line between the bases of the 
wings. The scutoscutellar ridge and its suture, however, are subject 
to much variation in form and degree of development, and they are 
sometimes obsolete or absent; but in general the V-shaped ridge and its 
suture are the most constant features of the wing-bearing plates and are 
present in some form in nearly all winged insects (Fig. 101, vs). 

A reversed natal suture occurs in some insects in which the true scuto- 
scutellar suture is obsolete or absent. In the Acrididae, for example, the 
usual V-ridge and its suture are partially suppressed (Fig. 101 C, vs), 
and the posterior part of the alinotum is marked by the line of a secondary 
ridge (rvs) of similar shape but having the apex directed posteriorly. 
The true scutellar region is thus divided into a median elevated shield- 
''haped area (Scl) and two lateral depressed areas (scl, scT). A similar 



180 


PRINCIPLES OF INSECT MORPHOLOGY 


topographical condition is even more strongly pronounced in the mesoi 
thorax of Hemiptera and Coleoptera. 

A transverse, or prescutal, suture (Fig. 100 A, ts), with its correspond- 
ing internal ridge (B, TR), is of frequent recurrence in many groups of 
insects. It lies in the anterior part of the alinotum and sets off a pre- 
scutum (Prsc) in the area immediately behind the antecostal suture. The 
prescutum is variable in size and shape. It is well developed in Plecop- 
tera, in some Orthoptera (Fig. 101 B, C), and in the mesothorax of 
Lepidoptera (G) and Coleoptera (H), but in other insects it is frequently 
very narrow (D, E, I) ; in Diptera it ends in a small lobe on each side 
of the notum before the wing base (D, E, e). Since the prescutal suture 
is often obsolete or absent, however, the prescutum may be but weakly 
defined or not distinguishable from the scutum (F, J). 



Fig. 100. — Diagrams showing the principal features of a generalized wing-bearing 
tergal plate. A, dorsal surface with sutures. B, ventral surface with corresponding 
ridges. Ac, antecosta; acs, antecostal suture; ANP, anterior notal wing process; atg, 
acrotergite; AxC, axillary “cord”; Em, lateral emargination ; Ph, phragma; PNP, posterior 
notal wing process; Pra, prealare; Prsc, prescutum; Rd, posterior marginal fold of alinotum 
continuous with wing margins; Sd, scutellum; Set, scutum; TR, transverse notal ridge; 
ts, transverse notal suture; VR, V-shaped, or scutoscutellar, ridge; rs, suture of V-shaped 
ridge; W, base of wing. 

A pair of convergent sutures, or notaulices, sometimes occurs in the 
anterior part of the alinotum. These sutures arise anterolaterally and 
extend posteriorly a varying distance, usually converging toward the 
median line of the back (Fig. 101 1, no); but the same sutures apparently, 
in some cases, may continue posteriorly to the transscutal suture (F, no) 
and thus divide the scutum into a median area {g) and two lateral areas 
{h, h). The convergent sutures are generally known as the “notauli” 
to systematists in Hymenoptera, which term is evidently a misspelling for 
notaulices (from aulix, aulicis, a furrow), but the same sutures are often 
called “parapsidal furrows,” and, again, many entomologists have 
regarded them as discontinuous median parts of the transverse prescutal 
suture turned posteriorly. If the convergent sutures are parts of the 
prescutal suture, the area between them is the prescutum, but in some 
insects, as in Tenthredinidae (Fig. 101 I), both the transverse prescutal 


THE THORAX 


181 


suture {is) and the convergent, sutures {no) are present, showing that the 
area between the latter belongs to the scutum. The development of the 
convergent sutures and their internal ridges is correlated with the pos- 
terior extension of the attachments of the dorsal wing muscles on the 
scutum (Fig. 12S B). 

A pair of lateral sutures, or parapsidal furrows, occurs frequently 
in the scutum of the mesothorax of Hj'mcnoptera; the sutures begin 



posteriorly usually at the transscutal suture and diverge forward a v^^ng 
distance in the lateral areas of the scutum (Fig. 101 F, p/). lese 
sutures, according to Tulloch (1929), are the sutures properly termed 
parapsidal furrows. The parts of the scutum lying laterad of them are 
the parapsides. In common practice, however, the anterior convergent 
sutures of Hymenoptera are often called the parapsidal furrows, and the 



182 


PRINCIPLES OF INSECT MORPHOLOGY 


areas laterad of them in Cynipoidea and Chalcidoidea are then designated 
the parapsides. In general usage the two terms, notaulices and parapsi- 
dal sutures, therefore, are usually synonymous since each may refer to 
the anterior convergent sutures. 

A median suture of the notum sometimes occurs in the anterior part 
of the scutum (Fig. 101 I, i) or extends through the entire length of the 
scutum (G, i). Internally it forms a median carina, which, when fully 
developed, extends like a ridgepole from the prescutum to the apex of 
the scutellum. 

A transscutal suture in most of the clistogastrous Hymenoptera, as in 
the honey bee (Fig. 101 J, 1), cuts completely through the posterior part of 
the scutum, setting off two posterolateral areas of the latter {set, set) 
from the major scutal area (Set). The parts of the alinotum separated 
by this suture are commonly termed “scutum” and “scutellum” by 
students of Hymenoptera, but it is clear that these areas are not identical 
with the scutum and scutellum of more generalized insects. The true 
scutoscutellar suture is well preserved in the Tenthredinidae (Fig. 101 1, 
vs) and may be present also in the higher Hymenoptera (J, vs) in conjunc- 
tion with the transscutal suture (1). 

A transscutellar suture cuts through the anterior part of the scutellum 
in higher Diptera (Fig. 101 E, /) between the lateral extremities of the 
scutoscutellar suture {vs). 

Various other sutures may occur in the alinotum, which are often 
characteristic of different orders, but which can be given no general 
names. In the metathorax of Coleoptera, for example, the lateral areas 
of the irregular scutum are partially divided by oblique sutures (Fig. 
101 H, j, j) into anterior and posterior parts {Set, Set). In the Diptera 
short lateral sutures or also longitudinal sutures occur in the mesoscutum 
{D, E, a, b, c, d), which give the sectional pattern characteristic of the 
wing-bearing notum in this order. 

A comparative study of the sutures and subdivisions of the alinotum 
brings out so many differences in these features between different orders 
that it becomes questionable if many of them are truly homologous 
structures. Aside from the antecostal suture, the only fairly constant 
character of a wing-bearing tergum is its division into two major parts 
by the suture of the V-shaped endotergal ridge, and even this feature 
is often obscured by a partial suppression of the ridge. The other ridges 
and sutures, producing various tergal subdivisions, are local adaptations 
to mechanical stresses and demands for flexibility in the wing-bearing 
plate, and a careful study of the wing mechanism would probably reveal 
the reason for them in each particular case. The structure of the tergum 
is often quite different in the two segments of the pterothorax according 
to the relative development of the two pairs of wings, and in most insecjS 



THE THORAX 


183 


with small or rudimentary hind wings the metatergum becomes much 
reduced. 

The Alar Margins of the Almoliim.—The lateral margins of the 
alinotum are specificallj'' modified in adaptation to the complex articular 
and flexor mechanisms of the \Ying bases. Very frequently each anterior 
lateral angle of the postcostal region of the notum is produced in a prealar 
bridge, or prealare (Fig. 96, Pra), that extends laterad or ventrad to the 
epistemum (Eps) and thus supports the notum anteriorly on the pleural 
wall of the segment. The wings arise from the scutoscutellar margins 
of the alinotum (Fig. 100 A, TF), which may be long or much shortened, 
and the posterior thickened edges, or "axillary cords” (AxC), of the basal 
wing membranes are always continuous with the posterior marginal fold 
of the scutellum (Rd). 

Each alar margin of the scutum presents typically an anterior notal 
wing process (Figs. 96, 100 A, ANP) and a posterior notal wing process 
(PNP). The anterior notal wing process is almost always present and 
supports the neck of the first axillary sclerite of the wing base (Fig. 
101 B, I, lAx). Immediately behind it is a deep emargination in the 
edge of the scutum (Fig. 100 A, Em). The posterior ■cing process usually 
gives support to the third axillary of the wng base (Fig. 122, 3 Ax) , but 
sometimes a fourth axillary intervenes between the third axillary and the 
notal margin, and in such cases a posterior wing process is usually absent. 
In the metathorax of some Orthoptera a special arm of the alinotum 
supports the vannal veins of the wings. 

The Pleuron of a Winged Segment. — The pleura of the wing-bearing 
segments do not differ fundamentally from the prothoracic pleura, but 
secondary differences between the two may be considerable on account of 
the degenerative tendency of the prothoracic pleura, and because of the 
special developments that take place in the ptero thoracic pleura. The 
pleura of the pterothorax are important elements of the wing mechanism, 
though, for the most part, their role is a passive one. The pleura show 
many minor variations in structure, and their areas may be variously 
broken up into secondary sclerites. In most cases it is difficult to discover 
the mechanical significance of these modifications, but their progressive 
development wdthin a family or order often furnishes a valuable clue to 
the relationships of genera and families. 

The principal alar functions of the pleuron in a winged segment are to 
furnish a fulcrum for the wing and to give attachment to the pleural 
rving muscles, though usually, as already observed, the pleuron also 
supports the tergal plates on the prealar and postalar arms of the latter 
(Fig. 96, Pra, Pa). The wing fulcrum, or pleural wing process (WP), 
typically has the form of a short, thick arm arising from the dorsal margin 
of the pleuron. The wing process is braced internally by the pleural 



184 


PRINCIPLES OF INSECT MORPHOLOGY 


ridge, which in the adult insect extends dorsally or obliquely from the 
coxal process of the pleuron to the wing process. The pleural suture is 
thus to be identified as the groove on the external wall of the pleuron {PIS) 
that extends between the articular processes of the wing and the leg. 
Usually the pleural suture takes a direct course between these two points 
(Fig. 102 D, PIS 2 ), but it may be irregular or angularly bent (E, PIS 2 ). 

The episternum and epimeron of the wing-bearing segments exhibit 
numerous variations in form and undergo various subdivisions into 
secondary sclerites, but their modifications are in general easy to follow. 
The most frequent type of subdivision divides the episternal and epimeral 
regions into dorsal and ventral areas. These are distinguished as the 
supraepisternum, or anepisternum, the infraepisternum, or katepisternum, 
the supraepimeron, or anepimeron, and the infraepimeron, or katepimeron. 
An anterior subdivision of the episternum is a preepisternum; if it is 
continuous with the presternum, the anterior pleurosternal piece thus 
set off is termed the prepectus. The precoxal and postcoxal areas of 
the pleuron (Fig. 96, Prcx, Pcx) are usually sclerotized in the wing- 
bearing segments, forming precoxal and postcoxal bridges to the sternum. 
Sometimes they are separated from the episternal and epimeral regions, 
but usually in adult insects they are united with the sternum. The 
postcoxal bridge is generally narrower than the precoxal bridge and is 
more frequently absent. 

The trochantin of the pterothoracic segments (Fig. 96, Tn) is best 
developed in the more generalized Pterygota, but it always shows a 
tendency toward reduction and is lost in the higher orders. 

The Epipleurites. — The chief distinctive feature of the pterothoracic 
pleura, aside from the presence of the wing processes, is the development 
and individualization of small sclerites beneath the wing bases, on which 
important muscles of the mngs are inserted. These plates may be 
termed the epipleurites, since they lie above the principal pleurites from 
which they are derived, though they have often been called the “parap- 
tera.” The anterior, or episternal, epipleurites are the iasalares; the 
posterior, or epimeral, epipleurites are the subalares. Generally there 
is but one basalare in each segment (Fig. 96, Ba) and one subalare (So), 
though each is sometimes double. 

The epipleurites appear to be derived by a secondary separation from 
the upper edges of the episternum and the epimeron. While the subalare 
is nearly always a distinct sclerite in adult insects, the basalare is fre- 
quently but an imperfectly separated lobe of the episternum (Fig. 99, Ba) 
or merely an area of the latter on which the anterior pleural wing muscles 
are attached. In nymphal Orthoptera neither the basalare nor the sub- 
alare is yet differentiated from the rest of the pleuron (Fig. 102 A, B), 
and both the anterior and posterior pleural wing muscles arise from the 



THE THORAX 


185 


upper edges of the latter, the first on the episternum, the second on 
the epimeron (C, ^E', 3E"). 

The Mesopleuron of Diptera . — In the higher Diptera the structure 
of the pleuron of the mesothorax becomes complicated by several unusual 
modifications. In the Tipulidae, however, the mesopleural elements are 
relatively simple and easy to identify. In Holorusia (Fig. 102 D), for 
example, the pleural suture (PlSf) takes the ordinary straight course 
from the base of the coxa to the wing process, and the only unusual 
features of the pleuron are the presence of an incomplete suture (o) 
separating the episternal region (Epsf) from the precoxal region (Prcx), 



Fig. 102. — Examples of simple and highly modified patterns of the pleural sclerites 
of wing-bearing segments. A, nymph of Melanoplus. B, nymph of Grylhis, meso- 
pleuron. C, inner view of same showing muscles. D, Holorusia grandis, mesopleuron. 
E. Tabanus atratus, mesopleuron and metapleuron. 


and the partial separation of the lower part of the epimeron {epmf) from 
the principal epimeral area {Epmf) above it. The meron of the middle 
coxa {Merf) is conspicuous by its size, but it is not detached from the 
rest of the coxa. 

In the higher Diptera, as exemplified by Tahanus (Fig. 102 E), the 
pleural suture of the mesothorax {PIS 2 ) is sharply flexed in two rec- 
tangular bends, and a long, membranous cleft (6) extends downward in 
the episternum, before the dorsal part of the pleural suture, from the wing 
process (WPf) to a horizontal episternoprecoxal suture (a). The 
episternal area is divided by this cleft (6) into a large anterior region 
{Epsf) separated from the precoxal area by the suture a, and into a 
smaller posterior region {epsf), most of which is ventral to the horizontal 


186 


PRINCIPLES OF INSECT MORPHOLOGY 


part of the pleural suture and continuous with the precoxal area (Prcx). 
The epimeron is also divided into a large supraepimeron {Epm-^ and a 
smaller infraepimeron {epm^. Finally, the meron of the mesocoxa 
{Mer 2 ) is completely detached from the rest of the coxa and is solidly 
incorporated into the pleural w^all, being united both with the epimeral 
plate above it and with the narrow postcoxal bridge {PCX 2 ) behind it. 
The ventral end of the subalar muscle of the wing, normally attached on 
the meron, as it is in Tipulidae, has migrated upward to the horizontal 
part of the pleural suture ; but the remotor muscle of the coxa retains its 
attachment on the meron and becomes an adjunct to the indirect elevators 
of the wings. These complex changes in the mesopleuron, together with 
the reduction of the metapleuron and the unusual modifications of the 
sterna (Fig. 95 B), give the thorax of the higher Diptera a very specialized 
type of structure. 

The Sternum of a Winged Segment. — The sterna of the wing-bearing 
segments show fewer special modifications than do either the terga or 
the pleura of these segments; their essential structure has been sufficiently 
described in the general discussion of the thoracic sterna (pages 166 to 
172), While the pterothoracic sterna differ in no important respect 
from the sternum of the prothorax, peculiarities of structure are likely 
to be more accentuated in them. Each is characterized principally by 
the greater size of the basisternal region on which the tergosternal muscles 
are attached. In the higher orders the second spinasternum is usually 
consolidated with the eusternum of the mesothorax and may become 
indistinguishable from the latter. 

6. THE THORACIC MUSCLES 

The thoracic muscles of all pterygote insects, excepting perhaps the 
Odonata, conform closely to one general plan of arrangement. The 
potential number of muscles in each segment appears to be limited, 
or, at least, the maximum number of muscles or of functional units of 
fiber bundles can be pretty definitely stated according to our present 
knowledge of the thoracic musculature in the principal orders of pterygote 
insects, though the full complement does not occur in any one group. 
^'^ariations in the muscle pattern, therefore, are the result principally 
of the absence of certain muscles, though a single muscle in one species 
may be represented in another by two or more bundles of fibers having a 
common point of insertion. The leg musculature varies according to the 
different tj'pes of movement in the coxae resulting from alterations 
in the coxal articulation on the body, the simpler types of leg musculature 
being evidently secondary modifications correlated with a limitation of 
the coxal movement. 



THE THORAX 


187 


Morphologically the usual thoracic muscles, as represented in an 
alate segment, may be classed as (1) dorsal muscles, (2) tergopleural 
muscles, (3) tergosternal muscles, (4) tergocoxal muscles, (5) pleuro- 
sternal muscles, (6) pleurocoxal muscles, (7) ventral muscles, (8) sterno- 
coxal muscles, (9) lateral intersegmental muscles, and (10) spiracular 
muscles. The musculature of the prothorax differs from that of a winged 
segment chiefly in the lack of the tergosternal muscles and of muscles 
in other groups that function principally in connection with the movement 
of the wings in the pterothoracic segments. 



Fig. 103. — The thoracic musculature, diagrammatic, showing most of the muscles 
known to occur in a wing-bearing segment. A, dorsal, ventral, tergosternal, and oblique 
muscles of right side, inner view. B, lateral and leg muscles of right side. A, 
dorsal muscles (1 A, oblique lateral dorsals; mA, longitudinal median dorsals) ; B, tergopleural 
muscles; C, tergosternal muscle; D, the wing flexor; E', basalar muscles {IE’, pleurobasalar; 
SE', sternobasalar; SE', coxobasalar) ; E", subalar muscles {IE", pleurosubalar; 3E", 
coxosubalar) ; F, oblique intersegmental muscle; G, pleurosternal muscle; H, ventral mus- 
cles; I, J, tergal promoter and remoter of coxa (upper parts cut off) ; K, L, sternal promoter 
and remotor of coxa; M, abductors of coxa; N, adductor of coxa. 


For a practical study of the thoracic musculature it will be found 
more convenient to classify the muscles in the following groups, the 
individual muscles of which are shown diagrammatically in Fig. 103, 
the more median muscles on the right side at A, the lateral muscles at B. 
The spiracular muscles, omitted here, will be described in connection 
with the spiracles. 

A. Dorsal Muscles . — The muscles of this group comprise longi- 
tudinal median muscles and oblique lateral muscles (Fig. 103 A, mA, lA). 
In the prothorax the principal dorsal muscles extend from the first 
phragma to the postoccipital ridge of the head (Fig. 87, DMcl), though 
others may go from the tergum to the head or from the tergum to the 


188 


PRINCIPLES OF INSECT MORPHOLOGY 


first phragma. In the pterothorax the median dorsals are usually highly 
developed (Figs. 97 D, DMcl, 103 A, mA), at least in the segment bearing 
the principal pair of wings (Fig. 97 E), and are attached on the phragmata 
or also on the alinotal and postnotal plates. They are reduced in wingless 
insects, or in insects with weak powers of flight, as in Isoptera, Blattidae, 
and Gryllidae. The oblique lateral dorsals, when present (Fig. 103A, lA), 
extend from the posterior part of the scutum to the following phragma. 
Though usually relatively small, these muscles are highly developed 
in the mesothorax of higher Diptera and are large and powerful, almost 
vertical, muscles in the mesothorax of some Homoptera (Fig. 128 B, lA). 

B. Tergopleural Muscles. — ^These muscles, found principally in the 
■wing-bearing segments, are highly variable in their development, and 
some or all of them may be absent. Four muscles of this group have been 
recorded in the mesothorax of different insects. One goes from the 
prealar arm of the tergum to the episternum (not shown in the figure), 
another (Fig. 103 B, 2B) from the lateral tergal margin to the basalare, 
a third (SB) from the tergum to the wing process. The fourth muscle 
(4 jB) extends from the posterior part of the scutum to the base of the 
pleural arm or the lower part of the pleural ridge. The last is an impor- 
tant muscle in Epheraerida, Plecoptera, Sialidae, Mecoptera, Trichoptera, 
Aphididae and is often two branched. 

C. Tergosternal Muscles. — Generally large muscles of the ptero- 
thorax in flying insects; attached above on the scutum, below on the 
basisternum anterior to the coxae (Figs. 103 A, 128 B, 130). Absent in 
weak-flying insects, and not represented in the prothorax. These muscles 
are the principal levators of the 'wings, being functionally antagonistic 
to the dorsal muscles in their action on the tergum. 

D. Axillary Muscles. — ^The muscles of the axillary sclerites of the 
wing base arise on the pleuron and are probably in origin tergopleural 
muscles since the ■wings are expansions of the dorsum. Two muscles 
occur in this group. One, known only in Diptera, is inserted on the first 
axillary. The other, present in aU ■winged insects (Fig. 103 B, D), is 
inserted on the third axillary and is the usual flexor of the wing (Fig. 128 
C, D) ; it is a simple or branched muscle arising on the upper part of the 
pleuron. 

E. Epipleural Aluscles. — The muscles comprised in this group are 
the lateral muscles of the mesothorax and metathorax attached dorsaUy 
on the epipleural sclerites (basalare and subalare). In the adult they 
are important muscles of the 'wings, but in nymphal stages they arise 
on the upper margins of the pleura, and two of them appear to be pri- 
marily coxal muscles (Fig. 102 C). The basalar muscles (Fig. 103 B, E') 
include a muscle from the episternum (IE'), a muscle from the sternum 
before the coxa (2E'), and a large muscle (3E') attached below on the 



THE THORAX 


189 


outer margin of the coxa before the pleural articulation of the latter. The 
first two of these muscles are variable in occurrence, either one or both 
being often absent; the third is a constant feature of the thoracic muscula- 
ture of winged insects. The subalar muscles {E") duplicate the basalar 
muscles in reverse order, one arising on the epimeron ilE"), another 
on the sternum, and the third {ZE") on the coxa behind the pleural 
articulation. The coxosubalar muscle is an important element of the 
■\ving mechanism; the postcoxal sternosubalar is highly developed in 
Ephemerida but has not been observed in other orders; the epimero- 
subalar muscle appears occasionally in various insects. 

F. Lateral Intersegmental Muscle. — An oblique muscle attached 
below on the sternal apophysis, dorsally on the anterior margin of the 
following pleuron or tergum (Fig. 103 A, F). This muscle is more 
commonly present in generalized insects and in larval forms; in adults 
it usually occurs only between the prothorax and the mesothorax, but a 
corresponding muscle is sometimes present between the mesothorax 
and the metathorax. 

G. Pleurosternal Muscles. — The muscle most commonly present in 
this group consists of short fibers connecting the opposed ends of the 
pleural and sternal apophyses on each side of the segment (Fig. 103 A, 
GO ; absent when the apophyses are united. In rare cases a muscle extends 
from the lower end of the pleural ridge to the sternal apophysis. 

H. Ventral Muscles. — ^Longitudinal or oblique horizontal muscles 
stretched between the eusternal apophyses, between the spinasternal 
spinae, and between the apophyses and the spinae (Fig. 103 A). The 
prothoracic muscles of this group are attached anteriorly on the head, 
usually on the tentorial bridge, or some of them on the cervical sclerites 
(Fig. 87). 

I. Tergal Promotor of the Leg. — Usually a single large muscle, some- 
times double, arising dorsally on the tergum, inserted below on the 
ventral end of the trochantin (Fig. 103 B, I) or on the anterior angle 
of the coxa if the trochantin is absent. 

J. Tergal Remotor of the Leg. — A single muscle, or a group of muscles, 
arising dorsally on the posterior part of the tergum, inserted ventrally 
on the posterior rim of the coxa (Fig. 103 B, J). 

K. Sternal Promotor of the Leg. — Origin on the sternum; insertion 
on the anterior part of the coxal base (Fig. 103 B, K). If the coxa turns 
on the pleural articulation alone, this muscle is an anterior rotator of the 
coxa. 

L. Sternal Remotor of the Legr.— Origin on the sternum, the sternal 
apophysis, or the spina; insertion on the posterior part of the coxal base 
(Fig. 103 B, L). This muscle, as the last, is a rotator of the coxa if the 
latter has a free movement on the pleuron only. 



190 


PRINCIPLES OF INSECT MORPHOLOGY 


M. Pleurocoxal Muscles— Usually two muscles, arising on the epi- 
sternum, inserted on the coxal base anterior to the pleural articulation 
(Fig. 103 B, IM, 2M). These muscles appear to be abductors of the 
coxa if the coxa has no sternal articulation; otherwise they are coxal 
promotors. 

N. Adductor Muscle of the Coxa. — A muscle present in insects lacking 
a sternal articulation of the coxa, arising on the sternal apophysis, inserted 
on the mesal margin of the coxa (Fig. 103 B, N). 

P. Exiracoxal Depressor of the Trochanter. — The depressor of the 
trochanter usually has one or more branches arising in the body segment 
bearing the leg (Fig. 117); generally there is a branch from the tergum 
(Fig. 115 A, 133c), another from the sternal apophysis (133cl), and 
sometimes one from the pleuron (Fig. 102 C, P). 

GLOSSARY OF TERMS APPLIED TO THE THORAX 

Names used in the present chapter, but not given in the following list, may 
be found in the glossaries of Chaps. Ill, V, IX, and X. 

Alinotum (.AN). — The wing-bearing plate of the dorsum of the mesothorax or 
metathorax of pterj'gote insects. 

Anapleurite (Apt). — The dorsal supracoxal sclerotization of a generalized thoracic 
pleuron. (Eupleuron.) 

Anterior Notal "Wing Process (ANP). — ^The anterior lobe of the lateral margin of 
the alinotum supporting the neck of the first axillary. (Vorderer Tergalhebel.) 

Basalare (Ba). — The epistemal epipleurite (sometimes double) giving insertion 
to the anterior pleural muscles of the wing; often represented by an undetached or 
partially detached lobe of the epistemum before the pleural wing process. (Epi- 
siermlgelenksluck, preparapteron.) 

Basistemum (Bs). — The principal area of the sternum anterior to the roots of the 
sternal apophyses or the sternacostal suture. (Sternannum.) 

Cervical Sclerites, Cervicalia (cw). — ^The sclerites of the neck, particularly one or 
two pairs of lateral neck plates (Kehlplatten) joining the head to the prothoracic 
episterna. 

Cervix (Cvx). — ^The neck; including probably the posterior nonsclerotized part of 
the labial somite and the anterior part of the prothorax. 

Coxopleurite (Cxpl). — The sclerite of a generalized thoracic pleuron adjacent to 
the dorsal margin of the coxa, bearing the dorsal coxal articulation; its anterior part 
becomes the definitive trochantin. (Evlrochaniin, Trochantinopleura.) 

Epimeron (Epm ). — The area of the pleuron posterior to the pleural suture, some- 
times divided horizontally into a supraepimeron, or anepimeron, and an infraepimeron, 
or kalcpimcTon. 

Epipleurites (Ba, Sa). — The basalar (Ba) and subalar (jSq) sclerites of a wing-bear- 
ing segment differentiated from the upper ends of the epistemum and epimeron, 
respectively. (Paraptera, Pleuralgelenkslucke.) 

Epistemum (Eps). — ^The area of the pleuron before the pleural suture and above 
the trochantin, sometimes divided horizontally into a supraepislernum, or anepister- 
rtum, and an infraepistemum, or katepistemwn. 

Eupleuron (Apl). — See anapleurite. 

Eustemum (ES). -The intrasegmental ventral plate of a thoracic segment, exclu- 
sive of the spinasternum, but usually including the stemopleurites. 

Eutrochantin. — See coxopleurite. 



THE THORAX 


191 


First Thoracic Spiracle (Sp2 ). — The spiracle of the mesothorax, often displaced 
into the posterior part of the prothorax. 

Fiu-ca (Fu). — The forked endosternal process of higher insects, formed of the 
sternal apophyses supported on a median inflection of the sternum. 

Furcasternum. — A distinct part of the sternum in some insects bearing the furca. 
(The term generally applied to the sternellum.) 

Interstemites {Isl). — Primary intersegmental sclerites of the venter, becoming the 
spinasterna of the thorax. 

Laterostemite (Ls). — The lateral part of a definitive thoracic sternum apparently 
derived from the ventral arc (sternopleurite) of the subcoxa. 

Mesothorax (Thi ). — The second segment of the thorax; bearing the first pair of 
wings in winged insects. 

Metathorax (Th,). — The third segment of the thorax; bearing the second pair of 
wings in winged insects. 

Notaulices (no). — Longitudinal furrows convergent posteriorly in the anterior 
part of the mesonotum of some insects. (Incorrectly spelled “notauli,” and some- 
times mistaken for the parapsidal furrows. Singular, notaulix.) 

Notum (T). — The tergum, or particularly the tergum of a thoracic segment. 

Parapsidal Furrows (p/). — Lateral grooves divergent anteriorly in the posterior 
part of the scutum of the mesothorax of some Hymenoptera. 

Parapsides. — Lateral areas of the mesoscutum in some Hymenoptera laterad of the 
parapsidal furrows. (Singular, parapsis.) 

Paraptera. — See epipleurites. 

Phragmanotum (PN). — See postnotum. 

Phragmata (Ph). — Plate-like apodemal lobes of the antecostae of the mesonotum, 
metanotum, and first abdominal tergum; the second and third carried by the post- 
notal plates of the pterothorax when the latter are separated from the following terga 
to which they normally belong. (Singular phragma.) 

Pleural Apophysis {PI A). — The internal arm of the pleural ridge. 

Pleural Ridge (PIR). — The endopleural ridge formed by the pleural suture, bracing 
the pleuron above the leg, or between the coxal articulation and the wing support. 

Pleural Suture (PIS). — The external groove of the pleural ridge, separating the 
episternum from the epimeron. 

Plemal Wing Process (TFP). — The wing support of the pleuron at the upper end 
of the pleural ridge. 

Pleuron {PI). — The sclerotization of the pleural area of a body segment, probably 
derived from the subcoxal part of the primitive limb basis. 

Postalar Bridge, Postalare {Pa). — A lateral extension of the postnotum of a 
wing-bearing segment behind the wing base, generally united with the epimeron. 
{Later opostnotum.) 

Postcoxal Bridge, Postcoxale {Pc). — ^The postcoxal part of the pleuron, often 
united with the sternum behind the coxa. 

Posterior Notal Wing Process {PNP). — A posterior lobe of the lateral margin of 
the alinotum supporting the third axillary sclerite of the wing base. {Hinterer 
Gelenkfortsatz . ) 

Postnotum, Phragmanotum {PN). — The postscutellar, phragma-bearing plate 
often present in the dorsum of an alate segment, derived from the anterior part of the 
following tergum. {Postscutellum.) 

Prealar Bridge, Prealare {Pra). — A lateral extension of the prescutal area of the 
alinotum before the wing base, sometimes connected with the episternum. 

Precoxal Bridge, Precoxale {Prcx). — ^The precoxal part of the pleuron anterior to 
the trochantm, asually continuous with the episternum, frequently united with the 
sternum, sometimes a distinct sclerite. 



192 


PRINCIPLES OF INSECT MORPHOLOGY 


Prepectus (Prp). — An anterior marginal sclerite of the sternopleural areas of a 
segment, set off by a transverse suture continuous through the sternum and epistema. 

Prescutal Suture {ts). — A transverse groove of the mesonotum or metanotum 
behind the antecostal suture, setting off a prescutum from the scutum, and forming 
internally a prescutal ridge {TR). 

Prescutum (Prsc). — The anterior area of the mesonotum or metanotum between 
the antecostal suture and the prescutal suture, when the latter is present. 

Presternum (Prs). — A narrow anterior area of the sternum sometimes set off from 
the basisternum by a submarginal suture of the eusternum. (Not the acrosternite.) 

Propodeum. — The first abdominal segment of clistogastrous Hymenoptera incor- 
porated into the thorax. {Median segment.) 

Prothorax (Thi). — The first segment of the thorax. 

Pterothorax. — The two wing-bearing segments, often closely connected or united 
with each other, 

Scutellum (Scl). — The area of the alinotum posterior to the suture of the V-shaped 
notal ridge, or the corresponding area when the ridge is incomplete or absent. 

Scutoscutellar Suture (vs). — The external suture of the V-shaped notal ridge of 
the alinotum, the arms divergent posteriorly, dividing the notum into scutum and 
scutellum. 

Scutum {Set). — The area of the alinotum anterior to the suture of the V-shaped 
notal ridge, or between this suture and the prescutal suture if the latter is present. 

Second Spiracle (Spa). — ^The metathoracic spiracle, located near the anterior 
margin of the raetapleuron, between the mesopleuron and the metapleuron, or in the 
posterior margin of the mesopleuron. 

Spina (Spn). — The median apodemal process of a spinasternum. 

Spinastemum (Ss). — One of the spina-bearing intersegmental sclerites of the 
thoracic venter, associated, or united, with the sternum preceding; a spinasternum 
may become a part of the definitive prosternum or mesosternum, but not of the 
metasternum. 

Stemacosta. — The transverse internal ridge of the sternal suture through the 
bases of the sternal apophyses. 

Stemacostal Suture. — The external suture of the stemacosta, separating the 
basisternum from the sternellum. 

Sternal Apophyses (iSA). — The lateral apodemal arms of the eusternum; in 
higher insects united on a median base, the whole structure forming the furca. 

Sternellum {SI). — The area of the eusternum posterior to the bases of the sternal 
apophyses or the stemacostal suture. 

Stemopleurite {Spl). — The infracoxal sclerotization of a generalized thoracic 
pleuron, generally united with the primary sternum in the definitive eusternal plate. 

Sternum {S, Sin). — Primarily the primitive sternum {Stn), or sclerotization of the 
true venter of a segment; secondarily the definitive sternum (<S), which in the thorax 
usually includes the stemopleurites and may include the following intersegmental 
spinasternum. 

Subalare {Sa). — The epimeral epipleurite giving insertion to the posterior pleural 
muscle of the wing. {Epimeralgelenkstiick, postparapteron.) 

Trochantin {Tn). — The precoxal sclerite of a thoracic pleuron derived from the 
anterior part of the primitive coxopleurite; usually articulated at its ventral end to 
the anterior margin of the coxa, and giving insertion to the tergal promotor muscle of 
the leg. 

V-shaped Notal Ridge (VfJ). — The V-shaped endoskeletal ridge of the mesonotum 
or metanotum, its arms divergent posteriorly, marked externally by the scutoscutellar 
suture. {V-Leiste.) 



CHAPTER IX 


THE THORACIC LEGS 

The appendages of insects that ordinarily serve as organs of terrestrial 
locomotion are the appendages of the three thoracic segments; but in 
some Apterygota and in the larvae of various pterygote insects the 
abdominal appendages also play a part in the locomotor function. In 
the present chapter only the general structure and the musculature of 
the thoracic legs will be considered; the legs of many insects assume 
various other functions than that of locomotion and are structurally 
modified accordingly. 


1. STRUCTtTRE OF THE LEGS 

In describing the structure and mechanism of the legs we shall limit 
the term “leg” to the free part of the appendage having the coxa as its 



base, since we need not be concerned here with the question of the sub- 
coxal origin of the pleuron. The surfaces of the legs are oriented for 
descriptive purposes when the limb is extended at right angles to the 
body; the preaxial surface is then anterior, the postaxial surface posterior, 
the outer surface dorsal, and the inner surface ventral. 

The Leg Segments. — The typical and usual segments of the insect 
leg (Fig. 104) are the coxa {Cx), one trochanter {Tr), the femur (Fm), 
the tibia (Tb), the tarsus {Tar), and the pretarsus {Ptar). In the Odonata 
two trochanteral segments are present (Fig. 109 C, 1 Tr, 2 Tr), but they 
are not movable on each other. 

The Leg Joints. — The joints of the legs are membranous rings of the 
leg wall between the cylindrical sclerotized areas that constitute the 
segments. The membrane of the joint is the articular corium. Some- 
times there are no contiguous points of articulation between adjoining 

.193 


194 


PRINCIPLES OF INSECT MORPHOLOGY 


segments; but usually one or two pairs of opposed articular surfaces limit 
the movement of the joint to that of a hinge. Hinged joints are therefore 
either monocondylic (Fig. 105 A) or dicondylic (B). A single articula- 
tion is typically dorsal; in dicondylic joints one articulation is anterior 
and the other posterior, except at the trochantero-femoral joint where the 
articulations if present are usually dorsal and ventral. The coxo- 




Fia. 105. — Diagrams of articular mechanisms at the femoro-tibial joint of a leg. A, 
monocondylic joint. B, C, dicondylic joint, end view and side view with levator and 
depressor muscles. 


trochanteral hinge is always dicondylic with an anteroposterior axis. 
In the telopodite, dicondylic hinges are characteristic of the legs of adult 
insects; monocondylic hinges are usual in the legs of larvae (Fig. 106), 
but in the larvae of Neuroptera and Trichoptera the femoro-tibial joint 
is dicondylic. 

The structure of the articulations between the leg segments varies 
much at different joints and at corresponding joints in different insects. 

Sometimes the opposing surfaces 
simply touch at their points. In 
other cases the articulation is of the 
ball-and-socket type, a condyle on 
one surface fitting into a socket on 
the other. In dicondylic hinges 
of this kind the two articulations 
are frequently reversed in struc- 
ture. An occasional, perhaps gen- 
eralized, type of articulation 
consists of a flexible sclerotic bar 
continuous from one segment to 
the other through the articular 
membrane. 

The Coxa. — In its more symmetrical form the coxa has the shape of a 
short cylinder or truncate cone (Figs. 104, Cx, 107 A), though commonly 
it is ovate and may be almost spherical. The proximal end of the coxa 
is girdled by a submarginal basicostal suture (Fig. 107 A, bes), which forms 
internally a ridge, or basicosta (Be), and sets off a marginal flange, the 
coxomarginale, or basicoxiie (Bex). The basicosta strengthens the base 



Fig. 100. — Thoracic leg of a caterpillar, 
anterior %’iew. 




THE THORACIC LEGS 


195 


of the coxa and is commonly enlarged on the outer wall to give insertion to 
muscles (B, C) ; on the mesal half of the coxa, however, it is usually weak 
and often confluent with the coxal margin. The trochanteral muscles 
that take their origin in the coxa are always attached distal to the 
basicosta. 

The coxa is attached to the body by an articular membrane, the coxal 
corium, which surrounds its base. It has almost always an outer articu- 
lation with the pleuron of its segment, and it may have an inner articula- 
tion with the sternum or with a laterosternal sclerite, as was observed 
in Chap. VIII. These two articulations are perhaps the primary dorsal 



Fig. 107. — Diagrams illustrating structural details of the coxa. A, lateral view 
of coxa. B, C, inner view of outer wall of basicoxal region. D, a coxa with the meron 
extended distally. Be, basicosta; hca, basicostal suture; Bex, basiooxite; c, pleural 

articular socket; Cx, coxa; exs, coxal suture; f, anterior coxotrochanteral articulation; 
Fm, femur; g, posterior coxotrochanteral articulation; Mer, meron; Tr, trochanter. 


and ventral articular points of the subcoxo-coxal hinge (Fig. 88, c, d). 
In addition, the insect coxa has often an anterior articulation with the 
anterior, ventral end of the trochantin (Fig. 91 B, e), but the trochantinal 
articulation does not coexist with a sternal articulation (A). The 
pleural articular surface of the coxa is borne on a mesal inflection of the 
coxal wall (Fig. 107 A, c). If the coxa is movable on the pleural articu- 
lation alone, the coxal articular surface is usually inflected to a sufficient 
depth to give a leverage to the abductor muscles (Fig. 114, M) inserted 
on the outer rim of the coxal base. Distally the coxa bears an anterior 
and a posterior articulation with the trochanter (Fig. 107 A, /, g). 

The outer wall of the coxa is often marked by a suture extending from 
the base to the anterior trochanteral articulation (Figs. 106, 107 A, 
108 A, cxs). In some insects the coxal suture falls in line with the pleural 
suture (Fig. 108 B), and in such cases the coxa appears to be divided into 
two parts corresponding to the episternum and epimeron of the plemon. 
The coxal suture is absent in many insects (Fig. 108 C). 

The inflection of the coxal wall bearing the pleural articular surface 
divides the lateral wall of the basicoxite into a prearticular part and a 


196 


PRINCIPLES OF INSECT MORPHOLOGY 


postarticular part, and the two areas often appear as two marginal lobes 
on the base of the coxa. The posterior lobe is usually the larger and is 
termed the meron (Fig. 107 C, Mer). 

Tlie meron may be greatly enlarged by an extension distally in the 
posterior wall of the coxa (Fig. 107 D, Mer ) ; in the Neuroptera, Mecoptera 
(Fig. 99), Trichoptera, and Lepidoptera, the meron is so large that the 
coxa appears to be divided into an anterior piece, the so-called “coxa 
genuina,” and the meron {Mer), but the meron never includes the region 
of the posterior trochanteral articulation, and the groove delimiting it is 
always a part of the basicostal suture (Fig. 107 D, hcs). A coxa with an 



Fig. 108. — Coxal structures of a grasshopper, Dissosteira Carolina. A, coxa and 
trochanter of first leg, anterior view. B, same of middle leg, with adjoining part of pleuron. 
C, coxa and reduced trochanter of hind leg, anterior view. D, inner view of articulation 
of middle coxa iidth pleuron. 


enlarged meron has an appearance similar to one divided by a coxal suture 
falling in line with the pleural suture (Fig. 108 B), but the two conditions 
are fundamentally quite different and should not be confused. 

The meron reaches the extreme of its departure from the usual condi- 
tion in the Diptera. In some of the more generalized flies, as in the 
Tipulidae, the meron of the middle leg appears as a large lobe of the coxa 
projecting upward and posteriorly from the coxal base (Fig. 102 D, Mer); 
in higher members of the order it becomes completely separated from the 
coxa and forms a plate of the lateral wall of the mesothorax (E, Mer). 
By this transposition of the meron, the remotor muscle of the coxa 
attaclied on it loses its function as a leg muscle and serves as a depressor 
of the tergum, thereby becoming an adjunct to the usual elevators of the 




THE THORACIC LEGS 


197 


wings. The meral plate in the thorax of the Diptera was long a puzzle to 
entomologists until its true nature was shown by Crampton and Hasey 
(1915) and by Crampton (1925, 1925a). 

The Trochanter. — The trochanter (Fig. 104, Tr) is the basal segment 
of the telopodite; it is always a small segment in the insect leg, freely 
movable by a horizontal hinge on the coxa, but more or less fixed to the 
base of the femur. When movable on the femur the trochantero- 
femoral hinge is usually vertical or oblique in a vertical plane, giving a 
shght movement of production and reduction at the joint, though only a 
reductor muscle is present (Fig. 109 A, R). In the Odonata, both 



Fig. 109. — The coxal and troohanteral musculature. A, diagram of inner view of 
posterior wall of coxa, trochanter, and base of femur, with typical musculature. B, 
trochanter and base of femur of an ichneumoid, Megarhyssa, showing basal subdivision 
(/m) of femur. C, corresponding part of the leg of a dragonfly larva, showing divdded 
trochanter (J Tr, ZTr) with reductor muscles of femur (S) in second segment. 

nymphs and adults, there are two trochanteral segments (C, ITr, 2Tr), 
but they are not movable on each other; the second contains the reductor 
muscle of the femur (F). The usual single trochanteral segment of 
insects, therefore, probably represents the two trochanters of other 
arthropods fused into one apparent segment, since it is not likely that the 
primary coxotrochanteral hinge has been lost from the leg. In some of 
the Hymenoptera a basal subdivision of the femur simulates a second 
trochanter (Fig. 109 B, /m), but the insertion of the reductor muscle (R) 
on its base attests that it belongs to the femoral segment, since as shown 
in the odonate leg (C), the reductor has its origin in the true second 
trochanter. 

The Femur. — ^This, the third segment of the insect leg (Fig. 104, Fm), 
is usually the longest and strongest part of the limb, but it varies in size 
from the huge hind femur of leaping Orthoptera (Fig. 116, Fm) to a very 
small segment such as is present in many larval forms. The volume of 
the femur is generally correlated with the size of the tibia.1 muscles 



198 


PRINCIPLES OF INSECT MORPHOLOGY 


contained within it, but it is sometimes enlarged and modified 
in shape for other purposes than that of accommodating the tibial 
muscles. 

The Tibia.— The tibia (Fig. 104, Th) is characteristically a slender 
segment in adult insects, only a little shorter than the femur or the com- 
bined femur and trochanter. Its proximal end forms a more or less 
distinct head bent toward the femur, a device allowing the tibia to be 
flexed close against the under surface of the femur. 

The Tarsus. — The tarsus of insects corresponds to the penultimate 
segment of a generalized arthropod limb, which is the segment called the 
propodite in Crustacea (Fig. 50 A, Tar). In adult insects it is commonly 
subdivided into from two to five subsegments, or iarsomeres (Fig. 104, 
Tar), but in the Protura (Fig. 53A), some Collembola, and most holo- 
metabolous insect larvae (Fig. 106) it preserves the primitive form of a 
simple segment. The subsegments of the adult insect tarsus are usually 
freely movable on one another by inflected connecting membranes, but 
the tarsus never has intrinsic muscles. The tarsus of adult pterygote 
insects having fewer than flve subsegments is probably specialized by the 
loss of one or more subsegments or by a fusion of adjoining subsegments. 
In the tarsi of Acrididae the long basal piece is evidently composed of 
three united tarsomeres, leaving the fourth and the fifth free (Fig. 118 A). 
The basal tarsomere is sometimes conspicuously enlarged and is distin- 
guished as the hasitarsus. On the imder surfaces of the tarsal subseg- 
ments in certain Orthoptera there are small pads, the tarsal pulvilK, or 
euplantulae (Fig. 118 A, o). The tarsus is occasionally fused with the 
tibia in larval insects, forming a tibiotarsal segment; in some cases it 
appears to be eliminated or reduced to a rudiment between the tibia and 
the pretarsus. 

The Pretarsus. — The terminal part of the insect leg in its usual form 
departs so far from the simple structure of a primitive end segment of the 
limb that entomologists generally have not understood its nature, 
though its morphological status has long been clear from the work of 
de Meijere (1901). 

In the majority of arthropods the leg ends in a simple clawlike seg- 
ment, which in the Crustacea is known as the dactylopodite (Fig. 50 A, 
Plar). The crustacean dactylopodite is provided with two muscles, a 
levator and a depressor, both arising in the tarsus, or propodite. In the 
Hexapoda, a simple dactylopodite-like end segment of the leg occurs in 
the Protura, in some Collembola, in the larvae of many Coleoptera, and 
in the larvae of Lepidoptera and Tenthredinidae (Fig. 106, Piar). In 
these forms it differs from the crustacean dactylopodite only in lacking a 
levator muscle and in having the fibers of the depressor muscle distributed 
in the tibia and the femur. 



THE THORACIC LEGS 


199 


In most other adult, nymphal, and larval insects, the pretarsus bears 
a pair of movable lateral claws situated upon its base and articulated 
dorsally to the end of the tarsus, and the body of the segment is reduced 
to a small median claw or a lobe-like structure. The median claw is 
weU preserved in the Lepismatidae (Fig. 110, C, D, dac) and the tendon 
of the depressor muscle arises from the ventral Up of its base (C, E, x). 
In Ja-pyx the base of the pretarsus forms a large plate ventrally (B, Utr) 
upon which is attached the depressor “tendon" (x), while its tip is 
reduced to a minute median claw (A, dac) lying dorsally between the 
bases of the lateral claws (C/n). In the so-called triungulin larvae of 
meloid beetles the apparent “lateral claws” of the feet are probably 
spines arising from the base of the median pretarsal claw, as are also the 
“lateral claws” of certain lampyrid larvae. 



Fia. 110. — Examples of the retention of tho median pretarsal claw {dac) in insects. 
A, protarsus of Japt/x, dorsal view. B, same, ventral view. C, pretarsus and end of 
tarsus of Lcpiama, lateral view. D, same, end view. E, median claw, or rudimentary 
daotylopodito, of Lcpisma, with "tendon” (x) of depressor muscle attached ventrally. 

The typical pretarsus, or terminal foot structure, in insects having 
true lateral claws (Fig. Ill A, B) arises from the end of the tarsus by a 
membranous base, upon which are supported the pair of lateral claws 
{Un) and a median lobe, the arolium (Ar). The claws are hollow multi- 
cellular organs and their cavities are continuous with the lumen of the 
pretarsus. Each claw is articulated dorsally to the unguifer (A, k), a 
median process of the distal end of the last tarsomere {Tar). The aro- 
lium, likewise a hollow lobe, is a direct continuation of the median part 
of the pretarsal base; it may be entirely membranous, or its walls may be 
partly sclerotized. On the ventral surface of the pretarsus is a median 
basal plate, the unguitractor (B, Utr), which is partly invaginated into 
the end of the tarsus {Tar). To its proximal end is attached the tendon- 
like apodeme {x) of the depressor muscle of the pretarsus, usually called 
the retractor of the claws. The unguitractor plate may be divided into 
two sclerites (C, Utr), or sometimes there is a sclerite distal to it distin- 
guished as the planta (Fig. 118 C, Pin). Lateral plates beneath the bases 
of the claws are termed auxiliae (Fig. Ill B, E, 1). In the Diptera two 
large lateral lobes of the foot, known as the pulvilli (D, E, Pv), arise from 
the auxiliary plates, one beneath the base of each claw, and there is 
commonly also present a median process, or evvpodiuni {Emp), arising 



200 PRINCIPLES OF INSECT MORPHOLOGY 

from the distal end of the unguitractor plate. The empodium may have 
the form of a spine, or it may be lobe-like and similar in form to the 
pulvilli. The arolium is rudimentary or absent in most Diptera except 
Tipulidae. 

All parts of the pretarsus are subject to much variation. The 
lateral claws are sometimes of unequal size, one becoming reduced or 



Fig. 111. — Examples of “foot” structures of insects. A, Periplaneta americana, 
dorsal view. B, same, ventral \'iew. C, Magidcada eeptendecim, ventral view. D, asilid 
fly, lateral view. E, same, ventral view. Ar, arolium (dorsal lobe); Emp, empodium 
(median ventral process or lobe); k, unguifer process of tarsus; I, auxilia; m, aroliar pad; 
71, accessory sclerites between claws; Pv, pulvillus (lateral ventral lobe); Tar, tarsus; Un, 
ungues (lateral pretarsal claws); Utr, ungitractor plate; x, "tendon” of depressor muscle 
of pretarsus (retractor of claws). 


occasionally obliterated, the result being a one-clawed foot, as in the 
Coccidae, Pediculidae, and mammal-infesting Mallophaga. Again, both 
claws may become very small, and both may be lacking. In the 
Thysanoptera the claws are minute and the foot consists principally of 
the bladderlike arolium. In some insects the arolium is hollowed beneath 
and acts as a vacuum cup to enable the insect to walk on surfaces too 
smooth or too hard for the claws to grasp. Other insects cling to such 
surfaces by means of a gummy liquid exuded from the ventral side of the 
foot. 


2. MUSCLES AND MECHANISM OF THE LEGS 

The muscles of the legs, as of any segmented appendage, are com- 
prised in two sets, namely, (1) muscles of the limb basis, or those that 
move the appendage as a whole, and (2) muscles of the telopodite seg- 



THE THORACIC LEGS 


201 


merits, or those that move individual parts of the limb. Muscles of the 
second class are usually named according to the limb segment on which 
they have their insertions, though they may be motors of tlie entire part 
of the appendage distal to the insertions. Most of the muscles of the leg 
segments of insects take their origin in the segment immediately proximal 
to the one on which they are inserted, but some of them arise in the second 
or third segment removed from the point of insertion. 

Mechanism of the Leg Base. — The possible movements of the coxa 
depend upon the nature of the coxal articulation with the body, which 
may have any one of three types of structure (Fig. 112). If the coxa is 
articulated to the pleuron only (A, c), it is free to make any movements 
that its musculature will impart to it; if, however, it is hinged between 



Fig. 112. — Diagrams illustrating throe types of coxal articulation. A, with pleural 
articulation (c) only. B, with pleural (c) and trochantinal (c) articulations. C, with 
pleural (c) and sternal (d) articulations. 


pleural and trochantinal articulations (B, c, e), its movements may be 
more limited, though the flexibility of the trochantin usually does not 
impose a rigid hinge motion on the coxa; but if the coxa is articulated to 
the pleuron dorsally and to the sternum ventrally (C, c, d), its move- 
ments are strictly limited to those of a hinge with the axis in a transverse 
plane. 

A typical insect coxa of the first or second types, having only a pleural 
articulation with the body or both pleural and trochantinal articulations 
(Fig. 113 A, c, e), is provided with muscles that arise on the tergum, 
muscles that arise on the sternum, and muscles having their origin on 
the pleuron. The dorsal muscles include tergal promotors (I) and tergal 
remotors (J). The sternal muscles comprise sternal promoters and 
remotors, or, functionally, anterior and posterior rotators (K, L), and also 
an adductor (N) . The pleural muscles include functional abductors (M) 
and, in the wing-bearing segments of adult insects, the muscles of the 
basalar and subalar sclerites (3E', 3E"), which serve as wing muscles. 

The muscles of the leg base are not necessarily inserted directly on 
the coxa itself. The tergal promotor, for example, is always inserted on 






202 


PRINCIPLES OF INSECT MORPHOLOGY 


the trochantin (Fig. 113 A, Tn), except when this sclerite is much reduced 
or is absent. Some of the other muscles are frequently inserted on 
apodemes that arise in the articular membrane between the coxa and the 
pleuron. In special cases, certain muscles inserted on the pleuron are 
evidently coxal muscles that have been transferred to the pleural wall. 
The presence of an articulation between the trochantin and the coxa 
does not usually result in an alteration of the coxal musculature. 

A coxa of the third type (Fig. 112 C), having its movements limited 
to those of a hinge on a transverse or transversely inclined axis by 



Fio. 113. — The coxal musculature. A, diagram ol typical musculature of a coxa 
■nith pleural and trochantinal articulations. B, muscles of the mesothoracic leg of a bee 
with pleural and sternal articulations. (See Fig. 103 B.) 



articulations with both the pleuron (c) and the sternum (d), is likely to 
have a more simple musculature than that of a coxa of the first or second 
t 5 qje. In the middle leg of a bee, for example (Fig. 113 B), the usual 
tergal musculature of the coxa is absent, though the coxa has anterior 
and posterior sternal muscles {K, L), a large two-branched pleural muscle 
{M), and a subalar muscle (3E"). This form of reduced coxal muscula- 
ture is probably a specialized condition in the higher insects, since the 
primitive musculature of the leg base appears to comprise both tergal 
and sternal muscles. In the prothorax of the bee a posterior tergocoxal 
muscle (tergal remotor, J) is present. 

If the coxa has no sternal articulation, the pleural articular surface 
on its base is usually more or less inflected mesally from the outer wall 
(Fig. 114, c), occupying thus a point approximately central in the plane 
of tlie leg base. By this device a leverage is given to muscles inserted 
to an}’’ side of the articular point. The attachments of the coxal muscles 
occur approximately at the opposite ends of two axial lines through the 
articulation, one longitudinal (b-b), the other transverse (c-c). The coxa, 
therefore, has movements of promotion and remoiion on a transverse 
axis, and movements of abduction and adduction on a longitudinal axis; 



THE THORACIC LEGS 


203 


while furthermore, because of the single point of articulation, it is capable 
also of a partial rotation on a vertical axis {d-d) tlirough the articular 
point (c). 

In comparative studies of insect musculature we must consider 
muscles as functional groups of fibers rather than as individual fiber 
bundles, for it is often found that a single muscle in one species is repre- 
sented by several muscles in another, and tliat, even in the same species, 
the number of muscles in a functional group varies in the different 


segments of the thorax. Thus 
while the coxal musculature may 
be represented diagrammatically 
in a simplified form, as in Fig. 
113 A, it must be borne in mind 
that at each cardinal point of the 
coxal base there may be attached 
several distinct fiber groups consti- 
tuting a functional unity. There 
can be little doubt also that 
between different species, or 
between different segments of the 
same species, homologous muscles 
may have quite different functions 
onlng to differences in the relations 
between the skeletal parts on which 
they are inserted. For this reason 
it is impossible to name insect 
muscles consistently in all cases on 
a basis of their functions. 

For an elementary study of the 
insect leg muscles it will be best 



d 


Fig. 114. — Dingratn of the possible axes 
of movement of a coxa having only a pleural 
point of articulation (c). b-b, a.xis of 
abduction and adduction (muscles M and i\f); 
c-c, axis of promotion and remotion (muscles 
I and J); d-d, axis of partial rotation (mus- 
cles K and L), 


to examine some particular species, and the leg musculature of the 
acridid Dissosieira Carolina is here given as fairly representative of the 


more generalized type of leg musculature in insects. The muscles of 
the legs of the grasshopper are essentially the same in each segment, with 
the difference only that a single group of fibers in one leg may be repre- 
sented by two or more groups in another, and that the anterior rotator is 
absent in the prothoracic leg. The following descriptions are based 


specifically on the musculature of the hind leg. 


Muscles of the Leg Base of Dissosteira.— The coxae of the grass- 
hopper are attached to the body by the pleural articulations only (Fig. 
108 D, c), though a small trochantinal plate is present at the base of 
each leg, the sclerite being best developed in the fore leg (A, Tn) and 
becoming successively smaller in the other two (B, C, Tn). 



204 


PRINCIPLES OF INSECT MORPHOLOGY 


Tergal Promotor of the Coxa (Fig. 115 A, 118).— k large muscle of 
the hind leg lying immediately posterior to the tergosternal muscle 
of the metathorax. Origin dorsaUy on the lateral area of the scutum; 
insertion ventrally on a stalked apodemal disc of the anterior angle 
of the coxa (D, F). In the prothorax this muscle is inserted on the 
ventral end of the trochantin, as it is in most cases in which the trochantin 
is not rudimentary. 

Tergal Remotors of the Coxa (Fig. 115 A, 119, 120). — Two bundles 
of fibers attached on the posterior part of the coxa. The larger anterior 
one (119) arises on the posterior margin of the scutal area of the meta- 
tergum and is inserted on an apodemal disc of the posterior inner angle 
of the coxal base (D, F, 119). The smaller posterior muscle (A, 120) 
lies close behind 119 and is inserted on a slender apodeme attached to the 
extreme posterior angle of the coxa (B, D, F, 120). 

The tergal promoter and remotors are clearly antagonists to each 
other because of their opposite relations to the pleural fulcrum of the 
coxa. 

Anterior Rotator of the Coxa (Fig. 115 D, E, F, 121). — A large muscle 
vdth fibers arising in two groups, one from the lateral part of the sternum 
before the base of the sternal apophysis, the other from the sternellar 
lobe behind the apophysis (E, 121)‘, all fibers converging to a common 
point of insertion on the mesal side of the anterior angle of the coxal 
base (D, E, 121). 

Posterior Rotators of the Coxa (Fig. 115 D, E, 122, 12S, 124). — A group 
of three muscles arising on the posterior surface of the lateral arm of the 
metasternal apophysis (E); all inserted posteriorly on the base of the 
coxa (D, E). 

In the mesothorax there is but a single anterior rotator and a single 
posterior rotator, the first arising on the sternellar lobe, the second on the 
mesosternal spina. In the prothorax the anterior rotator is absent, and 
the posterior rotator includes two muscles, one from the sternal apophysis, 
the other from the spina. Since the rotators lie approximately in the 
plane of the coxal base, it is clear that they must serve to give the coxa 
a partial rotation on the pleural articulation. 

Abductors of the Coxa (Fig. 115 C, 125, 126). — Two muscles arising 
on the episternum of the metathorax and inserted on the outer rim of the 
coxal base appear to belong to the abductor system of the hind leg. 
The first, however, a very small muscle (125), is inserted so far forward 
on the coxa that it probably functions as an accessory to the tergal 
promoter (A, 118). The second (C, 126) covers most of the inner face 
of the episternum and its fibers converge upon a slender apodemal stalk 
arising in the articular membrane at the base of the coxa close before 
the pleural articulation (C, D, F, 126). Because of the mesal inflection 



Fig. 115. — Musculature of the hind coxa of a grasshopper, Dissosteira Carolina, 113, 
tergosternal muscle; 114f wing flexor; 118, tergal promotor of coxa; 119, 120, tergal remotors 
of coxa; 121, sternal promotor (anterior rotator) of coxa; 122, 123, 124-, sternal remotors 
(posterior rotators) of coxa; 126, 126, abductors of coxa; 127, basalar-sternal muscle; 128, 
basalar-coxal muscle; 129, subalar-coxal muscle; 130, sternal adductor of coxa; 13Sc, 
ISSd, body branches of depressor of trochanter. 

Adductor of the Coxa (Fig. 115 C, D, E, F, lSO)—k flat muscle taking 
its origin on the posterior surface of the metasternal apophysis beneath 
the posterior rotators (E), and extending posteriorly and ventrally 



206 


PRINCIPLES OF INSECT MORPHOLOGY 


to the inner margin of the coxal base (C, D, E, F, ISO). This muscle is 
evidently antagonistic to the abductor {126) since the two are inserted 
at opposite ends of a transverse axial line through the pleural articulation 
(C, D, F). 

Two other muscles attached on the outer rim of the coxa, present m 
the mesothorax and metathorax but absent in the prothorax, are muscles 
of the epipleurites and function as wing muscles in the adult. These 
muscles in the metathorax are the following: 

Second Pronator-extensor of the Hind Wing (Fig. 115 B, 128). — A 
large muscle attached dorsally on the second basalar sclerite (2Bo) 
and ventrally on the base of the coxa anterior to the pleural fulcrum 
(D, F, 128). 

Depressor-extensor of the Hind Wing (Fig. 115 B, 129). — A very 
tliick muscle attached dorsally on the subalar sclerite {So) and ventrally 
on the coxal base posterior to the pleural articulation (D, F, 129). 

In the nymph the last two muscles take their origin on the dorsal 
edge of the pleuron, one on the episternum (Fig. 102 C, 3B'), the other 
(3E") on the epimeron, and they here evidently belong to the abductor 
system of the coxa. 

Muscles of the Telopodite. — The part of the leg beyond the coxa 
is the principal movable part of the limb. The coxo-trochanteral 
joint at its base is almost universally a dicondylic hinge with articula- 
tions anterior and posterior relative to the normal vertical plane of the 
shaft of the limb. Its musculature, therefore, consists of levator and 
depressor muscles inserted on the basal segment, which is the trochanter. 
The levator fibers arise entirely within the coxa (Fig. 109 A, 0); the 
depressor fibers include a coxal group (Q) and usually one or more 
groups (P) arising in the body segment supporting the leg (Fig. 117). 

Muscles of the Trochanter. — In the hind leg of Dissosteira there are 
two levator muscles of the trochanter (Fig. 116 A, 131, 132) arising 
dorsally in the base of the coxa and inserted on the dorsal lip of the base 
of the trochanter. The depressor muscles include two groups of fibers 
arising ventrally in the base of the coxa {133a), and three groups taking 
their origin in the body of the metathorax. Of the latter, two arise 
on the tergum (Fig. 115 A, ISSc, the second not shown), and the third 
{ISSd) on the sternal apophysis (E, ISSd). The fibers of all groups are 
inserted on the ventral lip of the trochanter and together constitute a 
powerful depressor of the telopodite (Fig. 117, P, Q). 

Muscles of the Femur. — In the hind leg of Dissosteira there is no 
movement between the small trochanter (Fig. 115 D, Tr) and the femur 
{Fjh), and consequently the femiu' has no muscles. IFhen the femur is 
movable on the trochanter, it is provided with a short redactor muscle 
arising in the trochanter and inserted on the posterior edge of the femoral 



THE THORACIC LEGS 


207 


base (Fig. 109 A, R). This muscle is present in the first and second legs 
of Dissosteira and imparts a slight rearward flexion to the femur on the 
dorsoventral trochantero-femoral hinge. 

Muscles of the Tibia. — The tibial muscles are the most important 
muscles of the hind legs in the Acrididae, since it is the strong and sudden 
movement of the hind tibiae on the femora that gives the grasshopper 
its power of leaping. The muscles occupy almost the entire cavity of 
each femur (Fig. 116) and determine the size and shape of the latter; 
they comprise levator and depressor groups of fibers. Most of the levator 
muscle consists of two large masses of short overlapping fiber bundles 
occupying the dorsal three-fourths of the femoral cavity (A, B, lS5a, 



Fig. 116. — Muscles of the hind leg of a grasshopper, Dissosteira Carolina. ISl, ISB, 
levators of trochanter; 133a, coxal branch of depressor of trochanter; 134, 136, levators of 
tibia; 136, depressor of tibia; 137, 138, levator and depressor of tarsus; 139, depressor of 
pretarsus (retractor of claws). 

IS 5b). They are attached to the lateral femoral walls on the spaces 
between the “fishbone” ridges that form the external sculptured pattern 
of the outer and inner faces of the femur; they are inserted on a flat 
apodeme that tapers distally to a thick stalk arising from the dorsal 
margin of the tibial base (A, ISdAp). Two small branches of the 
levator muscle arise in the distal part of the femur from the dorsal wall 
and are inserted on the base of the apodeme (A, 135c, the other not 
shown). 

The depressor of the hind tibia is a relatively small muscle with long, 
slender fibers arising in the ventral part of the femur (Fig. 116 A, B, lS6a) 
and converging to the sides of a tapering apodeme arising in the ventral 
membrane of the knee joint. The terminal straplike part of this apodeme 
slides over a strong internal process (A, o) near the end of the ventral 
wall of the femur. Two small accessory bands of depressor fibers arise 



208 


PRINCIPLES OF INSECT MORPHOLOGY 


distally from the dorsal wall of the femur and are inserted on the base 
of the depressor apodeme (lS6b, the posterior one not shown). 

In the fore- and middle legs of Dissosteira the relative size of the 
tibial muscles is the reverse of that in the hind leg, the depressor being 



Fig. 117. — Mechanism of the hind leg of a grasshopper. 0, levator of trochanter 
and femur; P, Q, depressors of trochanter and femur; S, T, levator and depressor of tibia; 
U, F, levator and depressor of tarsus; X, depressor of pretarsus. 

the stronger of the two. In these legs there is also a very small anterior 
levator, which is reduced to a fibrous strand in the hind leg (Fig. 116 A, 
134-). The mechanism of the tibial muscles of the hind leg is shown in 
Fig. 117. 



Pretarsus of a grasshopper, Dissosteira Carolina. A, tarsus and pretarsus 
disjointed, showing tendonlike apodeme OlOAp) of depressor of pretarsus. B, pretarsus 
and end of tarsus, dorsal rdew. C, same, ventral view. Ar, arolium; Pin, plants; Tar, 
tarsus; Uf, unguifer; Un, ungues; Ulr, unguitractor plate. 

Muscles of the Tarsus . — ^The tarsal muscles, a levator and a depressor, 
are both small and lie in the distal part of the long slender tibia, (Fig. 116 
A, 1S7, 13S), the first inserted on the dorsal lip of the first tarsomere, the 
second on the ventral lip. The other subsegments of the tarsus are 



THE THORACIC LEGS 


209 


never provided with muscles and are consequently not independently 
movable. 

Muscles of the Pretarsus . — The pretarsus of insects, as in the Chilopoda 
and Diplopoda, always lacks a levator muscle. The depressor forms 
the flexor, or retractor, of the claws, so called because it serves to flex 
the claws ventrally and proximally on the dorsal articulations of the latter 
with the distal end of the tarsus. Its fibers arise in the tibia and the 
femur and are inserted on a long tendonlike apodeme (Fig. 118 C, llOAp) 
arising from the unguitractor plate (Utr) in the base of the pretarsus, and 
extending tlirough the tarsus and tibia (A) into the femur. In each leg 
of Dissosteira the first branch of the flexor of the claws arises posteriorly 
in the basal part of the femur; the second arises in the proximal bend 
of the tibia (Fig. 116 A, 139b); the third {lS9c) on the inner wall of the 
basal third of the tibia. The extension of the claws is caused by the 
elasticity of the basal parts of the pretarsus supporting them. 


GLOSSARY OF TERMS APPLIED TO THE PARTS OF AN INSECT’S LEG 

The following terms are here defined as they are used in entomology; more general 
terms applied to the appendages are given in the Glossary of Chap. V. 

Arolium (Ar ). — The usual median lobe of the pretarsus, arising between the bases 
of the claws. 

Auxiliae. — Small plates beneath the bases of the pretarsal claws, bearing the 
pulvilli when the latter are present. 

Basicosta (Be ). — The proximal submarginal ridge of the inner wall of a leg segment. 

Basicostal Suture (bcs ). — The external groove of a leg segment forming the 
basicosta. 

Basicoxite (Bex ). — ^The usually narrow basal rim of the coxa proximal to the 
basicostal suture and its internal ridge. (Coxomarginale.) 

Basitarsus. — The proximal segment of the tarsus. 

Coxa (Cx ). — The basal segment of the leg articulating with the pleuron, or also 
with the sternum. 

Coxal Corium. — The articular membrane surrounding the base of the coxa. 

Coxomarginale (Bex ). — See basieoxile. 

Dicondylic Joint. — A joint with two points of articulation between the adjacent 
leg segments. 

Empodium (Emp ). — A median lobe or spine-like process arising ventrally between 
the bases of the pretarsal claws, usually from the unguitractor plate. 

Euplantulae. — Padlike structures on the ventral surfaces of the tarsal subseg- 
ments. (Tarsal pulvilli.) 

Femur (Fm ). — The third and usually the largest segment of the insect leg. 

Meron (Mer ). — The lateral postarticular area of the base of the coxa, in some 
insects greatly enlarged and extended distally in the posterior part of the coxa, but 
always proximal to the basicostal suture. 

Monocondylic Joint. — A joint with a single point of articulation between the 
adjacent leg segments. 

Planta. — A median ventral sclerite of the pretarsus distal to the unguitractor 
nlate. 



210 


PRINCIPLES OF INSECT MORPHOLOGY 


Pretarsus (Plar ). — The terminal parts of the leg distal to the tarsus, including 
median remnants of the dactylopodite, and the lateral claws, or ungues; in most 
larvae a simple clawlike segment. 

Pulvilli (Pv ). — ^Lateral lobes of the pretarsus arising beneath the bases of the 
claws. Ventral lobes of the tarsal subsegments (euplantulae) are sometimes called 
tarsal pulvilli. 

Tarsus (Tar ). — The fifth segment of the leg, usually divided into from two to five 
subsegments, or tarsomeres. 

Tibia (Tb ). — The fourth segment of the leg. 

Trochanter (Tr ). — The usual second segment of the insect leg, probably composed 
of two united trochanteral segments; in some cases (Odonata) showing a division 
between its component segments. 

Ungues (Un ). — The lateral claws of the pretarsus; usually called “tarsal” claws. 

Unguifer. — The median dorsal process on the end of the tarsus to which the pre- 
tarsal claws are articulated. 

Unguitractor Plate (Ulr ).- — The ventral sclerite of the pretarsus from which arises 
the tendonlike apodeme of the retractor muscle of the claws. 



CHAPTER X 


THE WINGS 

Insects differ from the other fl 3 dng animals in that their wings are 
structures superadded to the primitive motor equipment of their ances- 
tors. The birds and the bats, in acquiring the power of flight, have lost 
the use of a pair of limbs for other modes of locomotion, since their wings 
are the forelegs made over for purposes of flsdng. Insects thus seem 
related to the winged creatures of fiction, though the latter, it must be 
observed, are given wings quite irrespective of their anatomical possi- 
bilities of using them. And yet the wings of insects, as we shall presently 
see, when first acquired were probably outgrowths of the back incapable 
of movement. Certainly their evolution into organs of flight has involved 
much reconstruction in the thoracic segments to contrive a motor 
mechanism for them. 


1. ORIGIN AND EVOLUTION OF THE WINGS 


The oldest insects known from the fossil records lived in Carboniferous 
times, their remains being found in the lower beds of the Upper Carbonif- 



Fia. 119. — Examples of fossil insects with paranotal lobes on the prothorax. A, 
Stenodictya lobata. (From Brongnxart, 1890.) B, Lemmatophora typica. (From Tillyard, 
1928.) 


erous or Pennsylvanian period. These ancient insects had two pairs of 
fully developed wings, which differed but little in structure from the 
wings of modern insects (Fig. 119). Many of the Carboniferous insects, 
however, had in addition to the wings a pair of small, flat lobes {pnl) 
projecting laterally from the tergum of the prothorax, and these lobes 

211 


5:12 


PRINCIPLES OF INSECT MORPHOLOGY 


suggest that at an earlier period the wings themselves were developed 
from similar tergal lobes of the mesothorax and the metathorax. We 
may casualize the immediate ancestors of the winged insects, therefore, 
as creatures ha^’ing the body already differentiated into head, thorax, 
and abdomen and characterized by the possession of a series of three 
partly overlapping, fanlike extensions, or paranoial lobes, projecting on 
each side of the body from the thoracic terga. If the paranotal lobes, 
then, were the precursors of the wings, it seems most probable that they 
served as gliding organs, allowing their possessors to launch themselves 
into the air from an elevation and to sail off to some more distant 


objective. 

The structure and development of the wings of modern insects attest 
the origin of the wings from lateral folds of the tergal margins, for each 
T wing is essentially a hollow exten- 

^ ~ — sion of the body wall, the dorsal 

\ ; If lamina of which is directly contin- 

pnl t Me I — pj uous with the tergal plate support- 

ing it, while the ventral lamina is 
y^-Cx reflected into the lateral wall of the 
segment (Fig. 120). The pleural 
_ ^ ^ r plates of the thoracic segments 

a thoracic segment 'nith paranotal extensions must have been evolved for the 
of the tergum. purpose of Supporting the bases of 

the paranotal lobes from below. We can thereby understand why the 
pleura of adult pterygote insects have the same essential structure in all 
the thoracic segments, and why they differ so characteristically from the 
primitive pleurites of the Apterygota. The special features of the pleura 
in the wing-bearing segments are final adaptations to the later-developed 
mechanism for moiung the wings. 

The first step in the evolution of the paranotal lobes into organs of 
flight must have consisted in the acquisition of a line of flexibility in the 
base of each lobe. The longitudinal dorsal muscles (Fig. 103 A, A) 
could then, by arching the terga upward between the ends of the seg- 
ments, throw the vdng flaps downward, since the latter, in their recent 
capacity of glider lobes, were already substantially supported from below 
on the pleura. In order to give mechanical eflflciency to the muscles in 
arching the terga, however, movement between the tergal plates had 
first to be eliminated. Sclerotic continuity in the dorsum of the wing- 
boaring region has been acquired either by a union of the successive terga 
or b}"^ a forward extension of the acrotergal lips of the metatergum and 
first abdominal tergum into the regions of the intersegmental membranes 
(Fig. 85). 



THE WINGS 


213 


With the downstroke of the wings produced by an upward bend of the 
tergum, the upstroke must depend on some antagonistic force that will 
flatten the tergum. It is possible that, in the first place, the elasticity 
of the tergal plates sufficed to restore the normal contour of the latter; 
but most modern insects have special tergal depressor muscles in the 
two wfing-bearing segments. These muscles lie laterad of the longitudinal 
muscles; they are attached dorsally on the anterior lateral parts of the 
tergum and ventrally on the sternum before the coxae (Fig. 103 A, C). 
They are perhaps deri^^ed from the primitive lateral body muscles. 

The vdngs of insects are thus movable up and down by a relatively 
simple mechanism. Flight, however, is not to be achieved by the mere 
flapping of a pair of flat appendages. Forward motion in the air depends 
upon a more complex movement in the motor organs, involving a slight 
forward and rearward action of the wings and a partial rotation on their 
long axes. With the vdngs sufficiently flexible at their bases, and of a 
proper structure in their distal parts, these movements may result from 
the changing air pressure on their surfaces when they are \dbrated in a 
vertical direction. The stiffening of the anterior parts of the wings by a 
forw'ard crowding of the veins (Fig. 131), and the flexibility of the more 
weakly supported posterior areas, automatically gives a torsion to the 
wing planes in motion. But if the wings had to depend upon air pressure 
alone for the slant of their planes that gives the forw^ard impulse to the 
insect in the air, it is evident that the wings on opposite sides of the body 
would have ahvays approximately the same degree of movement. Con- 
trolled or differential action in the twm wings of a pair would then be 
impossible, and the insect wmuld have no poAver of directing its flight or of 
changing its course. Most modern insects, how'ever, do control efficiently 
their motions on the AAing, and many of them, besides being wmll able 
to direct their forAvard flight, can also fly sideAAise and backward without 
changing the position of the body, or they can hover at one point in the 
air. It is possible that some insects may shift their course by altering 
ttie posture Avhile flying, but it has been showm that steering, for the 
most part, is a function of the wings. 

The rotary movement of the wings is produced chiefly by powerful 
muscles lying against the pleural AA’^alls of the wing-bearing segments 
(Fig. 129, E', E"). These muscles are inserted usually on small sclerites 
situated immediately beneath the wings, respectively before and behind 
the Aving fulcrum (TFP), or in some cases on lobes of the dorsal margin 
of the pleuron. The principal pair of these so-called “direct wing 
muscles” in each side of the segment take their origins ventrally on the 
coxa {Cx), shoAA^ing that they are primarily leg muscles that have been 
given over to the serAuce of the wdngs. When insects in this way acquired 
the direct action of muscles on the Aving bases, they possessed a mechanism 



214 


PRINCIPLES OF INSECT MORPHOLOGY 


capable of controlling the movements of each wing separately and thus 
became endowed with the power of directive flight. 

2. DEVELOPMENT OF THE WINGS 

Since the insect wing is a flattened, double-layered expansion of the 
body wall, its own walls consist of the same elements as the body wall, 
namely, cuticula, epidermis, and basement membrane, and its lumen 
contains nerves, tracheae, and the body fluid, or blood. 

The wings of insects with incomplete metamorphosis grow externally 
in the same manner as do the legs, the mouth parts, and other appendic- 
ular organs. The wing buds appear first in the second or third instar 
of the nymph as hollow, flattened outgrowths of the body wall along the 
lateral margins of the dorsum in the mesothorax and metathorax. They 
increase in size at each moult, without much change in structure until 
they assume the adult form at the transformation to the imago. In 
insects with complete metamorphosis the wings develop during the larval 
stage beneath the outer cuticula, usually within pouches of the epidermis. 
The time of their first formation varies in different insects from a late 
embryonic period to the last larval instar. The internal wing buds are 
normally everted from their pouches during the prepupal period of the 
last larval stage, but they are then still covered by the loosened cuticula 
of the larva. When this last larval skin is shed, and the insect enters the 
pupal stage, the wings are first exposed as external organs. During the 
pupal stage they develop rapidly and then quickly take their final form 
when the insect issues as an imago from the pupal skin. 

In the very young wing bud the epidermis consists of upper and lower 
layers corresponding to the dorsal and ventral surface of the wing fold; 
but very soon the epidermal cells become elongate, and the inner ends of 
those in the opposing layers meet and unite. The fused basement 
membranes then become the so-called middle membrane of the wing. 
Along certain lines, however, the basement membranes do not come 
together; the channels thus left open, which are remnants of the primitive 
wing ca^^ty, determine the courses of the future veins of the wing. The 
channels contain the wing nerves, tracheae, and blood. In later stages of 
development, the epidermal cells condense along the vein channels and 
form here the thick cuticular layers that are to constitute the walls of the 
wing veins. When the wing development is completed, the epidermis has 
largely disappeared, and the mature wing is almost entirely a cuticular 
structure. Nevertheless, an active circulation of blood persists in the 
adult uing, observed in insects of most of the principal orders, and sense 
organs are of frequent occurrence on the wing surfaces. 

The histological changes in the growth of the wing are somewhat 
more complicated in the Holometabola, but the developmental processes 



THE WINGS 


215 


arc osscnlinily the same as in insects with a simpler metamorphosis. The 
most important, accounts of the development of the wings will be found 
in the papei-s by Wei.smann (1SG4), Gonin (1894), Mayer (1896), Corn- 
stock and Needham (1898-1899), Mercer (1900), W. L. Tower (1903), 
Powell (1903), IMar.shall (1915), Comstock (1918), and Kohler (1932). 

The origin and growth of the tracheae of the wings arc of much impor- 
tance in the study of the wing venation, because, in many cases, the 
tracheation of the j'oung wing serves as a key to the homology of the veins 
of the adult wing. 'J'he wing tracheae arise from a basal trachea, or two 
united tracheae, at the base of the wing bud, and in general are given off 
into the latter in two groujis, one anterior, the other posterior. In insects 
with incomplete metamoriihosis the tracheae take their places in the 
wings before the veins arc formed and thus appear to determine the 
courses of the veins. In the Holomctabola, howe\’er, the vein channels 
may be formed in advance of the tracheae, and, though in the Coleoptera, 
Ncuroptera, and Lej)idoj)tera each trachc.a is said to penetrate the vein 
corresponding to the one its homologuo occupies in insects with incom- 
plete metamorphosis, in the Trichoptera, Hymenoptcra, and Diptera 
the relations between the tracheae and the veins are not so clearly 
preserved. 

The internal wing l)ud.s of holometabolous larvae arc aerated first 
by a few simple tracheae, but in the later larval stages they are supplied 
with numerous bundles of tracheoles that grow directly from the walls 
of the primary wing trachea. The definitive wing tracheae are finally 
formed during the last larval stage and become functional in the pupal 
stage, wlien the earlier tracheae and tracheoles degenerate. From the 
walls of the definitive tracheae, finally, a second set of tracheoles is 
developed, and these tracheoles become functional at the change to the 
imago. 

The development of the wing tracheoles and tracheae was first studied 
by Gonin (1894); the details of the origin of the wing trachea from the 
basal trachea or tracheae of the wing in the principal groups of insects 
have been described by Chapman (1918), and the development of the 
wing veins of a cockroach by Beck (1920). 

3. THE STRUCTURE OF THE WINGS 

In studying the wings of insects we must give special attention to 
three features of their structure, namely, the articulation to the body, the 
veins, and the differentiation of the alar surface into wing regions. The 
veins serve to strengthen the wing and to adapt it to the movements 
demanded of an organ of flight. The articular parts furnish the basal 
structure in the wing necessary for the movements of flight in the distal 
area and constitute also the flexor apparatus in the wing-flexing insects. 



216 


PRINCIPLES OF INSECT MORPHOLOGY 


The wing regions are local differentiations of the wing area partly sub- 
serving the function of flight, but largely accessory to the act of flexion. 

The principal veins of the wings spring from the wing base, and most 
of them, except those of the posterior area, branch in varying degrees in 



Fig. 121. — Diagrams of wing venation. A, the archetype venation, with veins named 
according to the Comstoek-Needham system. {Adapted from Bradley, 1931, to indude 
three branches of cubitus.) B, the usual wing veins and axillaries as designated in the 
accompanying text. A, anal veins; Ax, axillary sclerites (first, second, third, and 
fourth): AxC, axillary cord; C, costa; Cu, cubitus; h, humeral cross-vein; HP, humeral 
plate; J, jugal veins; ff, jugal fold; M, media; m, m', median plates; MA, media anterior; 
m-m, median cross-vein; m-cu, mediocubital cross-vein; MP, media posterior; Pc, pre- 
costa; Peu, postcubitus (first anal); R, radius; r, radial cross-vein; 72,, radial sector; s, 
sectorial cross-vein; tg, rudiment of tegula; V, vannal veins (anal veins except the first); 
vf, vannal fold. 

the distal part of the wing. It is probable that all the diverse patterns 
of wing venation found in living and extinct insects have been derived 
from a single type of primitive venation ; but the true primitive venation 
is not actually known, because the oldest fossil insects yet discovered have 
a highly complex system of wing veins. The venation pattern given in 
Fig. 121 A represents the plan of venation which students of the wing 





THE WINGS 


217 


veins of insects regard as an ancient type from which the venation of 
modern insects has been derived, and it is therefore termed the archetype 
venation. 

The theoretically complete archetype venation (Fig. 121 A) includes 
the following veins, named according to the Comstock-Needham system 
of vein nomenclature: first, a small precosta (Pc) at the base of the wing; 
second, a costa (C), which is usually marginal in modern insects; third, a 
two-branched subcosta (Sc)-, fourth, a five-branched radius (72); fifth, 
a six-branched media (ilf); sixth, a three-branched cubitus (Cu); and, 
finally, a varying number of anal veins (A). 

The vein nomenclature given above is adopted in the present text 
with the exception that the anal veins are not recognized as a homogeneous 
group. The first anal vein (Fig. 121 A, lA) in more generalized insects 
is alwaj's associated at its base vdth the cubitus, and in the wings of 
many nymphal insects it is represented by a distinct trachea (Fig. 125 A, 
Pcu). The independence of the first anal from the other anals becomes 
an important feature in a study of the mechanism of the wings. For 
this reason it is here designated the postcubitus (Fig. 121 B, Pcu). The 
rest of the anals, which constitute a definite functional group of veins in 
generalized insects, associated with the flexor sclerite (3Ax) of the wing 
base, are distinguished as vannal veins (F) because the wing region 
containing them often forms a large fanlike expansion (vannus) of 
the posterior part of the wing. At the base of the wing, proximal to the 
vannal region, there is usually a small but variously developed lobe, the 
jugum, which may contain one or two jugal veins (B, J). The second 
principal branch of cubitus, the postcubitus, and the vena dividens some- 
times present in the fold between the postcubital and vannal veins are 
the first, second, and third plical veins of Forbes (1933). 

In the vings of modern insects (Fig. 121 B) the precosta does not 
appear, and the anterior fork of media (A, MA) is usually absent. The 
remaining veins in the wing are subject to many modifications in different 
groups of insects by the union of adjacent veins or by a partial or even 
complete suppression of certain veins, and the venational pattern may be 
further complicated by the addition of secondary veins. It often 
becomes, therefore, a difficult matter to identify with certainty the veins 
that are present, and the problem of determining the wing-vein homolo- 
gies in the various orders of insects has been a major subject in 
entomology. A help in the study of the venation of adult insects may be 
derived from an examination of the basal connections or association of 
the veins with small sclerites in the articular region of the wing. These 
sclerites, the pteralia, are present in the wings of all insects, but they are 
particularly developed in insects that flex the wings over the back when 
at rest. The sclerites have definite and constant relations both to one 



218 


PRINCIPLES OF INSECT MORPHOLOGY 


another and to the bases of the veins. This fact has long been known, 
but for some reason students of wing venation have made little use of the 
basal connections of the veins in adult insects for determining vein 
homologies. 

The Articulation of the Wings. — ^The various movements of the wings, 
especially in insects that flex the wings horizontally over the back when 
at rest, demand a more complicated articular structure at the wing base 
than a mere hinge of the wing with the body. Each wing is attached to 
the body by a membranous basal area, but the articular membrane con- 
tains a number of small articular sclerites, collectively known as the 
pteralia (Fig. 121 B). The pteralia include an anterior humeral plate 
(HP) at the base of the costal vein, a group of axillaries (Ax) associated 
Vvith the subcostal, radial, and vannal veins, and two less deflnite median 
plates (m, m') at the base of the mediocubital area. The axillaries are 
specifically developed only in the wing-flexing insects, where they con- 
stitute the flexor mechanism of the wing operated by the flexor muscle 
arising on the pleuron (Fig. 128 C, D). Characteristic of the wing base 
is also a small lobe on the anterior margin of the articular area proximal 
to the humeral plate (Fig. 121 B, tg), which, in the forewing of some 
insects, is developed into a large, flat, scale-like flap, the tegula, over- 
lapping the base of the wing. Posteriorly the articular membrane often 
forms an ample lobe between the wing and the body, and its margin is 
generally thickened and corrugated, giving the appearance of a ligament, 
the so-called axillary cord (Fig. 122, AxC), continuous mesally with the 
posterior marginal scutellar fold of the tergal plate bearing the wing. 

The articular sclerites, or pteralia, of the wing base of the wing-flexing 
insects and their relations to the body and the wing veins, shown dia- 
grammatically in Fig. 122, are as follows: 

The Humeral Plate (HP). — Usually a small sclerite on the anterior 
margin of the wing base, movably articulated with the base of the costal 
vein; greatly enlarged in Odonata (Fig. 123 B). 

The First Axillary (lAx). — This sclerite is the anterior hinge plate 
of the Vidng base. Its anterior part is supported on the anterior notal 
^^'ing process of the tergum (ANP) ; its posterior part articulates with the 
tergal margin. The anterior end of the sclerite is generally produced as 
a slender arm, the apex of which (e) is always associated with the base 
of the subcostal vein (Sc), though it is not united with the latter. The 
body of the sclerite articulates laterally with the second axillary. 

The Second Axillary (2Ax). — ^This sclerite is more variable in form 
than the first axillary, but its mechanical relations are no less definite. 
It is obliquely hinged to the outer margin of the body of the first axillary, 
and the radial vein (R) is always flexibly attached to its anterior end (d). 
The second axillary presents both a dorsal and a ventral sclerotization 



THE WINGS 


219 


ill the wing base; its ventral surface rests upon the fulcral wing process 
of the pleuron (Figs. 128 C, 129, 2Ax). The second axillary, therefore, 
is the pivotal sclerite of the wing base, and it specifically manipulates 
the radial vein. 

The Third Axillary {3Ax ). — The third axillary sclerite lies in the 
posterior part of the articular region of the wing. Its form is highly 
variable and often irregular, but the third axillary is the sclerite on which 
is inserted the flexor muscle of the wing (D). Mesally it articulates 


Rm 


Vn 


Fig. 122. — Diagram showing the articulation of the wing witn the alinotum, and the 
basal relations of the veins to the humeral plate and the axillary sclerites. (Lettering as 
on Fig. 121.) 

anteriorly (/) with the posterior end of the second axillary, and posteriorly 
(jb) with the posterior wing process of the tergum {PNP), or with a small 
fourth axillary when the latter is present (Fig. 121 B, 4Aa:). Distally the 
third axillary is prolonged in a process which is always associated vuth the 
bases of the group of veins in the anal region of the wing here termed 
the vannal veins (F). The third axillary, therefore, is usually the 
posterior hinge plate of the wing base and is the active sclerite of 
the flexor mechanism, which directly manipulates the vannal veins. The 
contraction of the flexor muscle (D) revolves the third axillary on its 
mesal articulations {b, /) and thereby lifts its distal arm; this movement 
produces the flexion of the wing. 

The Fourth Axillary (Fig. 121 B, 4Ax). — This sclerite is not a constant 
element of the wing base. When present it is usually a small plate 
intervening between the third axillary and the posterior notal wing 
process and is probably a detached piece of the latter. 

The Median Plates (m, m '). — These sclerites are not so definitely 
differentiated as specific plates as are the three principal axillaries, but 




220 


PRINCIPLES OF INSECT MORPHOLOGY 


nevertheless they are important elements of the flexor apparatus. They 
lie in the median area of the vdng base distal to the second and third 
axillaries and are separated from.each other by an oblique line (b/) which 
forms a prominent convex fold during flexion of the vdng. The proximal 
plate (m) is usually attached to the distal arm of the third axillary and 
perhaps should be regarded as a part of the latter. The distal plate 
(to') is less constantly present as a distinct sclerite and may be repre- 
sented by a general sclerotization of the base of the mediocubital fleld of 
the wing. t^Tien the veins of this region are distinct at their bases, they 
are associated with the outer median plate. 

The Wing Base of Ephemerida. — The masrflies, when at rest, bring 
the wings together vertically over the back, but they do not flex the wings 
in the sense of folding them horizontally. A flexor mechanism, therefore, 



Fig. 123. — The iving articulation in insects that do not flex the wings. A, wing base 
of a mayfly. B, wing base of a dragonfly. Ax, axillary region; AxP, axillary plate; 
HP, humeral plate. 


is not developed in the bases of the wings; and yet the structure of the 
articular areas of the wings (Fig. 123 A) is not radically different from 
that of the wing-folding insects. At the base of each wing of the mayfly 
there is a small humeral plate {HP) intermediating between the head of 
the costal vein (C) and a small tergal lobe of the body segment supporting 
the wing. In the axillary region there is a group of weakly deflned 
sclerites {Ax), w^hich in their arrangement and relations to the vein bases 
give a suggestion of the axillaries of the wing-folding insects. The 
posterior part of the axillary membrane in the mayfly has the usual form 
of a fold bordered by a corrugated thickening, or axillary cord {AxC), 
continuous with the posterior margin of the tergum. 

The Wing Base of Odonata. — ^The articular region of the wing of a 
dragonfly contains tw'o large, strongly sclerotized plates (Fig. 123 B). 
The anterior plate {HP) supports the costal vein by a small intermediate 
sclerite (c) at the base of the latter and thus corresponds to the humeral 


THE WINGS 


221 


plate of Ephemerida (A) and the wdng-flexing insects (Fig. 121 B, HP). 
The great enlargement of the humeral plate in Odonata is evidently a 
specialized feature of the flight mechanism m this group. The posterior 
plate of the dragonfly vdng (Fig. 123 B, AxP) carries the four basal shafts 
of the postcostal veins and hence may be termed the axillary plate, since 
it corresponds in position to the group of sclerites in the ephemerid wing 
(A, Ax) that appear to represent the axUlaries of the ndng-flexing insects. 
The humeral plate of the odonate wing is hinged to the anterior half of 
the lateral edge of the tergum (T) of the segment supporting the wing, or 
in some species to a distinct sclerite (a) of the tergum. The axillary plate 
is articulated to the posterior half of the lateral tergal margin opposite a 
deep membranous area of the latter. The pleural wing process support- 
ing the wing has two arms, one applied to the humeral plate, the other to 
the axillary plate. The basal plates of the dragonfly’s wing turn up and 
dowm on the fulcral arms w'hen the wings are hfted or depressed. The 
twm plates, how'ever, are slightly movable on each other, and, since the 
costal vein (C) is doubly hinged to the humeral plate by a small inter- 
mediary piece (c) at its base, the costal area of the w'ing can be quite 
freely deflected independent of the rest of the Aving area, w'hich is solidly 
supported on the axillary plate by the veins attached to the latter. 

The flight mechanism of the Odonata, including the structure of the 
wing bases and the attachments of the w'ing muscles, appears to be a special 
development of a more generalized structure, retained by the Ephemerida, 
from W'hich the w'ing mechanism of other insects has been evolved. 

The Wing Veins. — The usual veins of the w'ing, omitting the precosta 
of certain fossil insects, are shown diagrammatically at B of Fig. 121. 
Their characteristic features and basal connections are as follow's: 

Costa (C). — The usual first A'ein of the w'ing, commonly marginal, but 
sometimes submarginal; associated at its base w'ith the humeral plate 
(HP). The trachea of the costal vein is perhaps a branch of the sub- 
costal trachea. 

Subcosta (Sc). — The second vein of the wing, typically forked distally 
into two short branches (Sci, Sc^) ; associated at its base with the distal 
end of the neck of the first axillary (lAx). 

Radius (E).— The third and generally the strongest vein of the wing. 
Toward the middle of the wing it forks into a first undivided branch (i2i) 
and a second branch, called the radial sector (Ra), w'hich subdivides 
dichotomously into four distal branches (R2, Rz, Ri, Rs)- Basally the 
radius is flexibly united with the anterior end of the second axillary (2 Ax). 

Media (Af).— The fourth vein of the w'ing. In the archetype pattern 
(A) the media forks into two main branches, a media anterior (MA), wliich 
divides into two distal branches (MAi, MA^, and a median sector, or 
media posterior (MP), which has four terminal branches (Mi, Mz, 



222 


PRINCIPLES OF INSECT MORPHOLOGY 


Ms, Mi). In most modern insects (B) the media anterior has been lost, 
and the usual “media” is the four-branched media posterior with the 
common basal stem. In the Ephemerida, according to present inter- 
pretations of the wing venation, both branches of the media are retained, 
while in Odonata the persisting media is the primitive anterior branch. 
The stem of the media is often united with the radius, but when it occurs 
as a distinct vein its base is associated with the distal median plate (m') 
or is continuously sclerotized with the latter. 



Flo. 124. — Bases of the fore and hind wings of Pcriplaneta americana. 

Cubitus (Cu ). — The fifth vein of the wing, primarily two branched. 
The primary forking of the cubitus takes place near the base of the wing 
(Figs. 124 B, 125 A, B, C), forming the two principal branches {Cui, Cu^. 
The anterior branch may break up into a number of secondary branches 
(Figs. 124 B, 125 B), but commonly it forks into two distal branches 
(Fig. 121 B, Cuia, Cuib). The second branch of the cubitus (Cmz) iQ 
Iljunenoptera, Trichoptera, and Lepidoptera was mistaken by Comstock 
and Needham for the first anal, as has been shown by Tillyard (1919), 
Lameere (1922), Tanaka (1926), Imms (1931a, 1934), and others. Proxi- 
mally the main stem of the cubitus is associated with the distal median 
plate {m') of the wing base. 



THE WINGS 


223 


Postcuhitus (Pcm).— This vein is the first anal of Comstock and 
Needham (Fig. 121 A, lA), except where these writers wrongly identified 
the second branch of cubitus as the “first anal.” The postcubitus, 
however, has the status of an independent wing vein (B, Pcu) and should 
be recognized as such. In nymphal wings, as amply shown by Comstock 
(1918), its trachea arises between the cubital trachea and the group of 
vannal tracheae (Fig. 125 A, Pcu). In the mature wings of more gener- 
alized insects the postcubitus is always associated proximally with the 
cubitus (Fig. 124 B) and is never intimately connected with the flexor 
sclerite (3Ax) of the wing base. In Neuroptera, Mecoptera, and Tri- 
choptera the postcubitus may be more closely associated with the vannal 
veins (Fig. 125 C, Pcu), but its base is always free from the latter. The 
postcubitus is usually unbranched; according to Lameere (1922) it is 
primitively two branched. In a former paper the writer (1930) called 
this vein “second cubitus (2Cu)” and mistakenly regarded it as repre- 
senting the second branch of cubitus in Orthoptera. 

Vena Dividens. — This is apparently a secondary vein present in the 
hind wing of some Orthoptera (Figs. 124 B, 134 B, vd), developed in the 
fold (vf) that sets off the vannal region from the wing region before it. 

Vannal Veins (IF to nV). — The vannal veins are the anal veins that 
are immediately associated with the third axillary, and which are directly 
affected by the movement of this sclerite that brings about the flexion 
of the wings. In number the vannal veins vary from 1 to 12, according 
to the expansion of the vannal area of the wing. The vannal tracheae 
usually arise from a common tracheal stem in nymphal insects (Fig. 
125 A, V), and the veins are regarded by Lameere (1922) and Tanaka 
(1926) as branches of a single anal vein. Distally the vannal veins are 
either simple or branched. 

Jugal Veins (J). — ^The jugal lobe of the wing is often occupied by a 
network of irregular veins, or it may be entirely membranous; but some- 
times it contains one or two distinct small veins, the first jugal vein, or 
vena arcuata (Fig. 121 B, IJ), and the second jugal vein, or vena cardinalis 
( 2 /). 

Cross-veins. — All the veins of the wing are subject to secondary 
forking and to union by cross-veins. In some orders of insects the cross- 
veins are so numerous that the whole venational pattern becomes a close 
network of branching veins and cross-veins. Ordinarily, however, there 
is a definite number of cross-veins having specific locations as indicated 
at B of Fig. 121. The more constant cross- veins are the humeral cross- 
vein (h) between costa and subcosta, the radial cross-vein (r) between Ri 
and the first fork of Rs, the sectorial cross-vein (s) between the two forks 
of Rg, the median cross-vein (m^m) between M 2 and Ms, and the medio- 
cubital cross-vein {m-cu) between media and cubitus. 



224 


PRINCIPLES OF INSECT MORPHOLOGY 

The veins of the wing appear to fall into an undulating series of convex 
veins and concave veins, according to whether they have a tendency to fold 
up or down when the wing is relaxed. The basal shafts of the veins are 
convex, but according to Lameere (1922) each vein forks distally into an 
anterior convex branch and a posterior concave branch. Thus the costa 



Flo. 125. — Examples of wing venation. A, hind wing of nymph of Scudderia. (From 
Comstock, 1918.) B, hind -wing of a plecopteron, Isogenus. C, forewing of Panorpa. 

and subcosta are regarded as convex and concave branches of a primary 
first vein, R, is the concave branch of the radius, MP the concave branch of 
the media, Cui and Cu^ are respectively convex and concave, while the 
primitive postcubitus and the first vannal have each an anterior convex 
branch and a posterior concave branch. The convex or concave nature 
of the veins has been used as evidence in determining the identities of the 
persisting distal branches of the veins of modern insects, but it has not 
been demonstrated to be consistent for all wings. 



THE WINGS 


225 


The Wing Regions, — In tlio wings of fill inspcts wc must distinguish 
n basal articular area from the (rue ala, or distal exjinnsc of the wing 
containing the veins. Tlie wiiig l)ase of Odonata, as wc liavc observed, 
contains two large plates, an anterior humeral plate (Fig. 12.3 B, HP) 
supporting the costal vein, and a posterior axillary plate (AxP) support- 
ing the other veins. The structure here is probably a specialized develop- 
ment of a more generalized structure of the wing base in primitive insects. 
In the ICphemerida (A) the humeral plate is small, as it is in insects 
generall}', and the axillary region (.-lx) contains a group of indistinctly 
diflerentiated selerite.c. In (he wing-flexing insects tlie axillarj' scleritcs 
are well defined and individualized; the area containing them is a definite 
feature of (he wing base and may be termed the axillary region of (ho 
wing (Fig. 12G, .-lx). 

The (rue alar ar<'a of the wing is always more or less n.sj'mmetricnl 
in form. The contour of the front margin is difTerent from that of the 
hind margin, and the ^lattern of the anterior venation never matches with 
that of (he venation in (he posterior part of the wing. 3'hero is a tend- 
ency for the anterior veins to become thickened and crowded toward (ho 
forward margin in such a manner as to give great (t rigidity to the front 
half of (he wing, while the weaker posterior veins are more widely spaced 
and give flexibility to the rear half. The alar area thus becomes dilTcr- 
entiated into an anterior region (Fig. 12(>, Uin), which is activelj' elTectivc 
in flight, and a posterior more pa.«sive region (Fn)- "I'he anterior rigid 
part of the wing may be termed the remigium (from Latin, an oar). In 
more generalized slow-flying insects, the ijosterior flexible part of the 
wing is often enlarged to form a fanlike expansion of the wing and hence 
maj' be termed the vannus (from Latin, a fan). In the more specialized 
.swift-flying in.-'cets the vaunus is reduced; but .since it contains the veins 
connected with the fle.xor selerite of the wing ba.so and is therefore an 
e.s.scntial part of the flexor apparatus, the vaunus is seldom entireb'^ 
obliterated. Finally there is often developed at the base of the wing 
proximal to the v.annus ji membranous lobe of the wing, the ncala of 
I^Iartynov (192.5), commonlj’ called the jugum (Fig. 126, Ju) because 
that of the forewing in some insects serves to 3 'oke the two wings on each 
side with each other. At the posterior angle of the wing base there 
sometimes occurs a membranous lobe, or jjair of lobes, known as the 
alula, or calyplcr. 

The three regions of the alar surface are commonly separated by lines 
of folding in the wing membrane. I’his is true jiartlcidarl}’’ when the 
wings are wide and cannot be j)laccd flat over the body in the flexed 
position. There occurs then between the remigium and the vannus, or 
appro-ximately separating the.se regions, a plica vannalis, or vannal fold 
(Fig. 12G, vf). This fold cither allows the vannus to take a horizontal 



226 


PRINCIPLES OF INSECT MORPHOLOGY 


position over the back, while the remigium slopes downward on the side, 
or it enables the vannus to be folded beneath the remigium in the flexed 
wing. The jugum, when well developed, is likewise separated from the 
vannus by a line of folding, the plica jugalis, or jugal fold {jf), and in the 
flexed wing the jugum is usually turned up or down on the inner edge of 
the vannus. The vannal fold, called also the '"anal furrow,” does not 
occur at exactly the same place in the wings of all insects, as will be 
noted in special examples to be described later, and in narrow-winged 
insects it may be eliminated. The wing regions are particularly distinct 



Fig. 126. — Diagram ol the wing regions in wing-fie.xing insects. Ax, axillary region; bf, 

basal fold; jf, jugal fold; J-u, jugum; Rm, remigium; Vn, vannus; vf, vannal fold. 

in insects having a large vannus, and especially in those that plait the 
vannus when the vdng is flexed. 

The Axillary Region. — The region containing the axillary sclerites 
(Fig. 122) has in general the form of a scalene triangle (Fig. 126, Ax). 
The base of the triangle (a-5) is the hinge of the wing with the body, 
the apex (c) is the distal end of the third axillary sclerite (Fig. 122, c) ; the 
longer side (Fig. 126, o-c) is anterior to the apex. The point d on the 
anterior side of the triangle marks the articulation of the radial vein with 
the second axillary sclerite (Fig. 122, d). The hne between d and c 
(Fig. 126) is the plica hasalis (bf), or fold of the wing at the base of the 
mediocubital field (Fig. 122, bf). 

The Remigium. — The vdng region anterior to the vannal fold (Fig. 
126, Rm) is the part of the mng chiefly productive of the movements of 
flight, since it is directly affected by the motor muscles of the wing. 
Wflien the vannal fold has the usual position anterior to the group of 
vannal veins (Fig. 121 B, vf), the remigium contains the costal, sub- 
costal, radial, medial, cubital, and postcubital veins. In the flexed wing 
the remigium turns posteriorly on the flexible basal connection of the 
radius with the second axillary (Fig. 122, d), and the base of the medio- 
cubital field is folded medially on the axillary region along the phca basalis 
(b/) between the median plates (m, m') of the wing base. 



THE WINGS 


227 


The Vanmis . — The vannal fold typically occurs between the post- 
cubitus and the first vannal vein (Figs. 121 B, 122, vj). In Orthoptera 
it usually has this position (Fig. 124 B, 134 A, B, vf). In the forevung 
of Blattidae, however, the only fold in this part of the wing lies imme- 
diately before the postcubitus (Fig. 124 A, 2/). In Plecoptera the vannal 
fold is posterior to the postcubitus (Fig. 125 B, vJ), but proximally it 
crosses the base of the fii'st vannal vein. In the cicada (Fig. 127 A) the 
vannal fold lies immediately behind the first vannal vein (IF). These 
small variations in the actual position of the vannal fold, however, do 
not affect the unity of action of the vannal veins, controlled by the 
flexor sclerite (3.4 a:), in the flexion of the wing. In the hind wings of 
most Orthoptera a secondary vena dmdens forms a rib in the vannal 
fold (Figs. 124 B, 134 B, D, vd). 

The vannus is usually triangular in shape (Fig. 126, Fn), and its 
veins typically spread out from the third axillary hke the ribs of a fan. 
Some of the vannal veins may be branched, and secondary veins may 
alternate with the primary veins (Fig. 134 B, a, h, c). The vannal region 
is usually best developed in the liind wing, in winch it may be enlarged 
to form a sustaining surface, as in Plecoptera and Orthoptera. The great 
fanlike expansions of the hind wings of Acrididae (Fig. 134 B) are clearly 
the vannal regions, since then veins are all supported on the third axillary 
sclerites of the wing bases, though Martynov (1925) ascribes most of the 
fan areas in Acrididae to the jugal regions of the wings. The true jugum 
of the acridid wing is represented only by the small membrane {Ju) 
mesad of the last vannal vein. The jugum is more highly developed in 
some other Orthoptera, as in the Mantidae. In most of the higher 
insects vdth narrow wings the vannus becomes reduced (Figs. 125 C, 
127 C), and the vannal fold is lost, but even in such cases the flexed wing 
may bend along a line between the postcubitus and the first vannal vein. 

The Jugal Region, or Neala . — The jugal region of the vdng (Fig. 126, 
Ju) is usually a small membranous area proximal to the base of the 
vannus strengthened by a few small, irregular veinlike thickenings; but 
when well developed it is a distinct section of the wing (Figs. 124 A, 125 C, 
127 A, D, Ju) and may contain one or two jugal veins (Figs. 121 B, 127 D, 
IJ, 2J). When the jugal area of the forewing is developed as a free 
lobe, it projects beneath the humeral angle of the hind wing and thus 
serves to yoke the two wings together. In the Jugatae group of Lepi- 
doptera it bears a long fingerlike lobe. The jugal region is termed the 
neala (“new wing”) by Martynov (1925), because it is evidently a 
secondary and recently developed part of the wing. 

The Alula . — At the posterior angle of the wing base in some Diptera 
there is a pair of membranous lobes {squamae, or calypteres) known as the 
alula. The alula is well developed in the house fly (Fig. 127 C, c, d). 



228 


PRINCIPLES OF INSECT MORPHOLOGY 


The outer squama (c) arises from the wing base behind the third axillary 
sclerite (3Aa:) and e^ddently represents the jugal lobe of other insects 
(A, D) ; the larger inner squama (d) arises from the posterior scutellar 
margin of the tergum of the wing-bearing segment and forms a protective, 
hoodlike canopy over the halter. In the flexed wing the outer squama of 
the alula is turned upside down above the inner squama, the latter not 



]?IG. 127. — Examples of wing venation. A, Magicicada septendedm, hind wing, 
extended. B, section of same along line a-b when folded. C, Musca domeslica, wing and 
calypteres. D, Epicauta pcnnsylvanica. 

being affected by the movement of the wing. In many Diptera a deep 
incision of the anal area of the wing membrane behind the single vannal 
vein sets off a proximal alar lobe distal to the outer squama of the alula. 

4. THE WING MUSCLES 

The movements of the wings in the majority of insects are accom- 
plished by five pairs or paired sets of muscles in each alate segment. 
These muscles are the dorsal muscles (Fig. 103, A), the tergosternal muscles 



THE WINGS 


229 


(C), the axillary muscles (D), the basalar 7nuscles {E'), and the suhalar 
muscles {E"). The dorsal and tergosternal muscles are often called the 
“indirect wing muscles,” and the axillary and epipleural muscles the 
“direct wing muscles,” but, strictly spealdng, onl}’- the axillary muscles 
in most insects are attached directly on the wing bases. 

In addition to the muscles listed above as specific wing muscles, it 
is probable that most of the segmental and intersegmental muscles of the 
pterothorax that are not leg muscles have some action in relation to the 
mng movements. Particularly the tergopleural muscles (Fig. 103 B, B), 
which extend from the tergum to the basalare, to the wing process, or 
to the epimeron, must exert some controlling influence on the movement 
of the tergum. Those inserted on the basalare undoubtedly have a direct 
action on the wings ; a large tergobasalar muscle present in some Diptera 
sharply extends the wing in a horizontal plane. Since however, the 
tergopleural muscles are highly variable and are not constantlj’- present 
in the wing-bearing segments, they will not be considered in the following 
general discussion of the wing muscles. 

The Dorsal Muscles. — These muscles are the ordmary longitudinal 
muscles of the back, which, in the usual secondary segmentation of the 
body (Fig. 37), extend from the antecosta of one tergum to that of the 
next. In the vdng-bearing segments of most insects the dorsal muscles 
are differentiated into median longitudinal muscles (Fig. 128 A, mA) and 
lateral oblique muscles {lA). 

The median dorsal muscles are usually greatl}' enlarged in the wing- 
bearing segments, and their expansion is accommodated by the develop- 
ment of phragmatal lobes on the antecostae of the mesotergum, the 
metatergum, and the first abdominal tergum, but frequently also their 
dorsal fibers encroach upon the postcostal surface of the almotum and 
on the precostal surface of the postnotum (Fig. 128 A). The longitudinal 
dorsal muscles are the principal depressors of the wings, since, by their 
contraction, they arch the wing-bearing terga upward between the ends 
of the segments and thus deflect the vings on the pleural fulcra (Fig. 131 
C). The action of the dorsal muscles as whig dejiressors, however, 
depends on an obliteration of the dorsal intersegmental membranes, a 
condition that has been brought about either b}’’ a fusion of the consecu- 
tive terga or by a forward extension of the precostal lips of the meta- 
thoracic and first abdominal terga (Fig. So) to form the phragma-bearing 
postnotal plates of the mesothorax and metathorax, respectively. The 
dorsal muscles are most highly developed in the segment bearing the 
principal pair of wings; the}'^ are usual] 3 ' reduced or absent in a segment 
of which the wings are small or are used for other purposes than that of 
flight. With insects such as Isoptera, Blattidae, and Gr}dlidac having 
weak powers of flight, the dorsal muscles are verj* small in both segment.': 



230 


PRINCIPLES OF INSECT MORPHOLOGY 


of the pterothorax, but they are also reduced in the strong-flying Odonata, 
in which the wings are moved by the lateral thoracic muscles. 

The lateral oblique dorsal muscles (Fig. 128 A, lA) arise on the pos- 
terior part of the scutum and are inserted posteriorly on the succeeding 
phragma laterad of the bases of the median dorsals. Usually these 
muscles are relatively small, and they are not always present; in function 
they probably supplement the tergosternals in their downward pull on 
the tergum. In the mesothorax of the cicada and in the Diptera the 



^^ew. B, mesothorax of Magicicada, median dorsals {mA) removed, shoTving almost 
vertical position of large lateral dorsals i^A). C, mesothorax of Dissosteira with basalar 


(E')t subalar {E"), and wing flexor muscles (Z)) of right side, mesal view. 


oblique dorsal muscles are unusually large (B, lA) and assume a position 
so nearl}’- vertical, by reason of the great size of the second phragma (2Ph), 
that they become powerful adjuncts of the tergosternal muscles (C) as 
depressors of the tergum. 

The Tergosternal Muscles. — ^These muscles lie to the sides of the 
median dorsal muscles in the anterior part of the segment (Figs. 103 A, 
128 B, C). They are attached dorsally on the anterior lateral areas 
of the tergum, and ventrally on the basisternum before the coxae. There 
maj' be one or several pairs of them in each segment. Functionally the 
tergosternal muscles are antagonists of the longitudinal dorsals, since 
by contraction they depress the tergum and thereby elevate the wings on 
the pleural fulcra (Fig. 131 A). These muscles have no representatives 
in the prothorax, and they may be absent in the pterothorax of insects 
of weak flight. 


THE WINGS 


231 


In the Diptera a third pair of muscles, lying between the anterior 
tergosternals and the posterior oblique dorsals, becomes secondarily 
levators of the wings. The muscles of this pair are the normal tergal 
remotors of the middle legs inserted on the meral lobes of the coxae. 
In the higher Diptera, however, the mesothoracic meron is detached 
from the rest of the coxa and becomes solidly incorporated into the lateral 
v.'all of the thorax (Fig. 102 E, Mer). The tergal remoter of the coxa 
is thus anatomically transferred from the leg and given over functionally 
to the service of the wing, since, by the loss of movement at its lower end, 
it becomes a depressor of the tergum. 

The Axillary Muscles. — The only muscles attached directly on the 
wing bases, in insects other than Odonata, are muscles arising on the 
pleuron and inserted on the first and third axillary sclerites. 

A muscle of the first axillary is known to occur only in Diptera. In 
a syrphid fly this muscle consists of two parts, one arising on the epi- 
sternum, the other behind the pleural ridge, both inserted on the inner 
margin of the first axillary. A pull on the muscle turns the axillary 
upward on its tergal articulations, which is the usual action of the first 
axillary during flexion of the wings. 

The muscle of the third axillary (Pig. 103 B, D) is present in all the 
wing-flexing insects, since it is the effector of the flexion movements 
of the wing. The muscle arises on the pleuron, but it is variable in size 
and distribution. Typically it consists of a single bundle of fibers 
attached on the pleural ridge (Fig. 128 C, D), but it may comprise 
several branches arising on the episternum, the pleural ridge, and the 
epimeron. Distally the flexor muscle is inserted on the base of the third 
axillary sclerite (Fig. 122, 3 Ax). Its contraction revolves the third 
axillary dorsally and inward on the proximal articulations of the latter 
(6, /) and thus turns the alar area of the wing posteriorly on the axillary 
region by a flexure along the line of the plica basalis (bf) . 

A muscle corresponding to the wing flexor is well developed in each 
wing-bearing segment of zygopterous Odonata. It arises on the pleural 
ridge and is inserted posteriorly on the axillary plate. 

The Basalar Muscles. — The muscles of the basalar sclerites, or of 
the basalar lobe of the episternum, usually include two muscles on 
each side, but sometimes three are present, or again only one. The 
first of the potential three muscles of this group arises on the episternum 
(Fig. 103 B, IB'), the second {2E') arises on the sternum or the precoxal 
bridge of the pleuron or occasionally on the trochantin (Gryllus), the third 
(3fl(') arises on the outer rim of the coxa anterior to the pleural coxal 
articulation. The last muscle appears to be a pleural leg muscle that 
has secondarily become a wing muscle in the adult by reason of the 
intimate connection of the basalare with the humeral angle of the wing 



232 


PRINCIPLES OF INSECT MORPHOLOGY 


(Figs. 128 C, 129, 130, a). The basalar muscles of the adult winged 
insect function as depressors of the costal margin of the wing during 
flight and as extensors of the flexed wing, for which reasons they may be 
termed the pronator-extensor muscles of the wing. 

In Odonata there are two anterior wing muscles arising on the lower 
edge of the episternum and inserted by long tendons directly on the large 
humeral plate of the wing base (Fig. 123 B, HP) . There are no epipleural 
sclerites in the dragonflies, and no pleurocoxal muscles are associated 
with the wing mechanism. 



Fig. 129. — Diagram of the pleural mechanisms of the wing. 


The Subalar Muscles. — In most insects there is but a single subalar 
muscle, usually of large size (Fig. 128 C, SE”), lying against the epimeral 
wall of the pleuron on each side of each wing-bearing segment, which 
is attached ventrally on the meron of the coxa. Associated with this 
muscle, however, there is sometimes, as in Gryllidae, Trichoptera, and 
Lepidoptera, a second muscle (Figs. 103 B, 129 IE") arising on the 
epimeron and inserted on the posterior part of the subalare or on a distinct 
second subalar sclerite (Gryllus). This muscle is a counterpart of the 
first basalar muscle (Fig. 103, IE') arising on the episternum. A subalar 
muscle corresponding to the sternal muscle of the basalare {2E') is 
known to occur only in Ephemerida; it is here a large muscle of the 
mesothorax arising medially on the sternum behind the coxae and 
inserted dorsally on the subalar region of the pleuron. In the mesothorax 



THE WINGS 


233 


of the higher Diptera the single large subalar muscle arises on the lower 
part of the epimeron dorsal to the meron, but this muscle is probably 
the usual subalar-coxal muscle transposed from the displaced meron 
to the pleural wall. The subalar muscles serve to extend and to depress 
the wing because of the close connection of the subalar sclerite with the 
second axillary sclerite of the wing base (Figs. 128 C, 129, b). They 
may be called, therefore, the depressor-extensor muscles of the wing. 

In the Odonata two posterior pleural wing muscles take their origins 
on the ventral edge of the epimeron in each alate segment and are inserted 
directly on the axillary plate of the wing 
base (Fig. 123 B, AxP). 

6, THE WING MOVEMENTS 

The insect wing is movable on the 
body by the flexibility of its basal con- 
nections noth the tergal plate and with the 
pleural wall of the segment, but it is 
definitely hinged to the tergum by the first 
and third axillary sclerites (Fig. 122, lAx, 

3Ax) or by the fii-st and fourth (Fig. 

121 B, lAx, i:Ax) if a fourth axillary is 
present. The ving, therefore, is capable 
of responding only to the up-and-down 
movements of flight on its extreme base 
line. Most of the other movements of 
flight, as well as the movements of flexion 
and extension, depend on the flexible 
connections of the veins with the articular 
sclerites, and on the interaction of the 
articular sclerites themselves. The mo- 
tions of insects^ wings fall into two distinct categories ; those of one 
include the movements of flight, those of the second embrace the move- 
ments of flexion and extension. 

The Movements of Flight. — The movements of the wing that make 
flight possible consist of an upstroke, a downstroke, a forward movement, 
a rearward movement, and a partial rotation of each wing on its long axis. 

The Upstroke of the Wings.— The elevation of the wings in flight 
is produced, as we have seen, by the simple device of depressing the 
tergum of the segment bearing the wings (Fig. 131 A), the action being 
the result of a contraction of the vertical tergosternal muscles (C), 
assisted in some cases by the oblique dorsal muscles and in Diptera by 
the remotors of the coxae. The mechanism of the upstroke, therefore, 
is simply that of a lever of the first order, the fulcrum being the pleural 



section of a 'svinged segment, anterior 
view, showing basalar mechamsm of 
extension and anterior deflection of 
the m'ng. 



234 


PRINCIPLES OF INSECT MORPHOLOGY 

wing process (TfP) upon which the base of the wing rests. The tergo- 
sternal muscles are often large and powerful, suggesting that the upstroke 
of the wings is an important contributant to the force of flight. 

The Downsiroke of the Wings.— The depression of the wings is not 
the work of a single set of muscles. It results in part from the restoration 
of the dorsal curvature of the back by the contraction of the longitudinal 
dorsal muscles (Fig. 131 C, A), which are the segmental antagonists of 
the tergosternal muscles; but probably an important effector of the wing 
depression in most insects is the posterior pleural muscle or muscles 



Fig. 131. — Diagrams of successive positions of the wings in flight and the corresponding 

movements of the tergum. 

(Figs. 128 C, 129 dE") inserted on the subalar sclerite {Sa). The subalar 
sclerite being in immediate connection (fc) with the second axillary of 
the wing base (2 Ax), a pull upon the subalar muscle strongly depresses 
the wing. 

The Anteroposterior and Rotary Movements of the Wings. — The partial 
rotation of each wing on its long axis is a part of the anterior and posterior 
movements and is accompanied by changes in the position of the plane 
of the ving surface during the upstroke and the downstroke caused by 
pressure of the air. It was formerly supposed that the torsion of the 
wings, including the horizontal and rotary movements, is entirely the 
result of changing air pressure on the flexible posterior areas of the wings 
as the latter are vibrated in a vertical direction. There is no doubt 
that the vings do respond by a differential action in their planes to air 
pressure alone, but it is also true that each wing is partially revolved at 
the base by the muscles of its motor mechanism. The muscles that 




Tim vriKGR 


235 


produce this movomont arc undoubtedly the muscles of the basnlar 
and subalar sclcrites. The first (Fig. 130, E'), pulling downward on the 
bnsalarc turn this sclorite inward on the upper edge of the cpistcr- 
num (Pf), and the connection (n) of the basalnre with the humeral angle 
of the wing base deflects the anterior part of the wing as it turns it slightly 
forward. The mechanism of anterior dcnectiou, including the basalar 
sclerite and its muscle or muscles, has been called the pronator apparatus 
of the wing. The movement of anterior deflection accompanies the 
depression of the wing (Fig. 131 C). 

The reverse movement, or tlie combined rearward motion and pos- 
terior deflection of the wing accompanying the upstroke (Fig. 131 A), 
is prohably caused largely by air pre.ssure on the expanded, flexible 
posterior area of the wing surface; but it is likely that the tension of the 



Flo. ms. — Curves (io^rrilvcfl on ti rnovinr recorder by the wine tii> of u stntionnry blow fly 
mntinf; tbe wine inovenionte of fliRlit, (From Ilillrr, 1911.) 

subalar muscles (Figs. 12S C, 129, E”), exerted on the second axillary 
sclorite (2/1t) posterior to the pleural fulcrum, contributes to the posterior 
deflection of the wing during the up.slroke. 

The ^ying Motion in Flight . — The motion of each wing in flight is the 
resultant of its several elemental movements. During the downstroke, 
the wing goes from above downward and forward; its anterior margin is 
deflected and its posterior area turns upward (Fig. 131 C). During the 
upstroke, tiic wing goes upward and relatively backward, and its posterior 
surface is deflected (A). 

As a result of the compound motion of the vibrating insect wing, the 
tip of the wing, if the insect is held stationary, describes a curve having 
the form of a figure 8. This fact has long been known from direct 
observation on insects in which the figure described by the vibrating 
wings is made visible in strong light by bits of gold leaf attached to the 
ving tips (Marcy, 1809, 1874). The wing motion, however, has been 
studied more accurately by mechanical devices in which a graphic record 
of the wing movements is obtained, as in the experiments of Marcy 
(18G9a) and of Ritter (1911), .showing that the wings of an insect in 
motion describe a series of open loops (Fig. 132), the distance between the 





236 


PRINCIPLES OF INSECT MORPHOLOGY 


loops depending on the speed at which the insect flies. The wing move- 
ments have also been recorded by cinematographic methods (see Marey, 
1901; von Lendenfeld, 1903; BuU 1904; Voss, 1913, 1914). The rotary 
movement of the wings is most accentuated in swift-flying insects, such 
as the dragonflies, bees, and flies, which have relatively narrow wings; 
in slower flying insects with broad wings, such as the grasshoppers and 
butterflies, the up-and-down movement is the principal one. 

The Rate of the Wing Vibration . — ^The rapidity of the wing motion 
varies much in different species of insects. Landois (1867) deduced from 
the pitch of the sound made by insects in flight that the house fly makes 
352 wing strokes a second, a bumble bee 220, and the honey bee, when at 
its best, 440, though when tired its hum indicates a speed of only 330 
beats a second. Marey (1869a) obtained graphic records of the wing 
beats on a revolving cylinder, and he gives 330 wing strokes a second for 
the house fly, 240 for a bumble bee, 190 for the honey bee, 110 for a wasp, 
28 for a libellulid, and 9 for the cabbage butterfly. Voss (1914), however, 
calculating the rate of the wing motion from series of moving picture 
photographs, obtained in most cases lower figures ; the honey bee, by his 
test, making 180 to 203 wing strokes a second, the house fly from 180 to 
197, the mosquito from 278 to 307, while various other insects have 
mostly a slower rate. In general it may be said the flies and bees have 
the highest speed of wing movement, other insects, by comparison, being 
slow of flight and correspondingly slow in wing motion. The lowest 
records of speed are obtained from the butterflies and moths, the cabbage 
butterfly making at best about 9 strokes a second, some of the noctuid 
moths about 40, though the spliinx moths, on the other hand, are swift 
fliers and move the wings at a high rate of speed. The student will find 
summarized statements of the recorded rates of the wing strokes in 
insects given by Voss (1914) and by Prochnow (1924, 1925). It must 
be recognized, however, that experimentally obtained records at best 
tell only what the insects did under the conditions of the experiment. 

The Movements of Flexion and Extension. — The movements by 
which the wings are folded after flight, or extended preliminary to flight, 
are executed too rapidly to be observed closely in a living insect; but 
the action of a wing and the operation of the flexor mechanism can be 
well studied in freshly killed specimens. A grasshopper, a bee, a fly, 
or almost any insect sufficiently large will answer the purpose, but the 
grasshopper, or particularly the scorpionfly Panorpa, will be found to be 
a very suitable subject. If the wing of a fresh specimen is slowly folded 
posteriorly over the back and then brought forward into the position of 
flight, the accompanying movements of the vein bases on the articular 
sclerites and the movements of the sclerites on one another can be 
obser^'ed. From the action of the parts in a dead specimen the 



THE WINGS 


237 


probable working of the flexor mechanism in the living insect can be 
deduced. 


We have seen that the axillary sclerites are contained in an axillary 
region of the wing base, which is approximately triangular (Fig. 133 A, 
Ax), the apex of the triangle (c) being formed by the outer end of the 
tliird axillary sclerite (Fig. 122, ZAx). The costal vein (C) alone has no 
connection with the axillary triangle, its base being associated with the 
humeral plate (HP). The subcosta (Sc) has a loose attachment (e) 


with the head of the first axillary, 
and the radius (R) is flexibly con- 
tinuous by its base (d) with the 
second axillary. The vannal veins 
(IV-nV) are closely associated with 
the outer end of the third axillary. 
The median and cubital veins (M, 
Cu, Pcu) have no direct connection 
with the axillary sclerites, but their 
bases, when distinct, are either 
associated with the second median 
plate (mO or more or less united 
in the corresponding area of the 
wing when this plate is absent. 
The base of the mediocubital field, 
therefore, abuts upon the basal fold 
of the wing, or plica basalis (6/), 
which forms the hinge line between 
the two median plates (m, m') when 
these plates are present as distinct 
sclerotizations. 

The essential skeletal element of 
the flexor mechanism is the third 
axillary. This sclerite is typically 
Y-shaped in form (Fig. 122, 3Aa;) 
inasmuch as it consists of a distal 



Fig. 133. — Diagrams of the typical folding 
of a wing during flexion. A, the wing 
extended and flat. B, the wing partly flexed 
by dorsal revolution of axillary area on its 
base (a-b). C, the fully flexed wing. (For 
lettering see Fig. 126.) 

stalk, the outer end of which (c) is 


associated with the bases of the vannal veins, and presents two proximal 


arms, the posterior one of which articulates with the tergum (6), while 
the anterior usually articulates with the posterior end of the second 
axillary sclerite (/) . The flexor muscle of the wing (D) is inserted on the 
base of the third axillary in the crotch between the two basal arms of 


the latter. 


Flexion of the Wings . — Flexion begins with a relaxation of the extensor 
muscles, which allows each wing to turn a little posteriorly. This 
automatic preliminary movement of the wing produces a strong convex 



238 


PRINCIPLES OF INSECT MORPHOLOGY 


fold at the base of the mediocubital field along the line of the plica basalis 
(Fig. 122, hj), wliich is between the two median plates (m, m') if these 
plates are present. At the same time, the movement revolves the third 
axillary sclerite (3 Ax) upward on its basal articulations (b, /). The 
insertion point of the flexor muscle (D) on the third axillary is thus 
turned dorsad and mesad of the axis of the basal hinge line of the sclerite, 
and the muscle, having now gained a purchase on the latter, is able by 
contraction to continue the revolution of the sclerite, turning it dorsaUy 
and mesally until it is completely inverted and reversed in position. 
The movement of the third axillary brings with it directly the vannal 
region of the wing (Fig. 133 A, Vn), the base of which is lifted and carried 
horizontally against the side of the back (B), while indirectly also it 
turns the remigial region (Rm) posteriorly on the articulations of the 
subcostal and radial veins with the first and second axillaries, producing a 
convex fold along the plica basalis (Fig. 122, bf), at the base of the 
mediocubital field. 

Since the first median sclerite of the wing base (Fig. 122, m) is usually 
attached to the distal arm of the third axillary, the rotation of the flexor 
sclerite (3Ax) has also a direct effect on the mediocubital field and brings 
about the folding along the plica basalis (bf) between the two median 
plates. With insects in which the vannal area of the wing is reduced, 
the action of the third axillary is principally on the mediocubital field 
tlwough the first median plate. By the revolution of the third axillary, 
the fold of the plica basalis is accentuated as the first median plate (m) 
is turned vertically on its hinge wdth the second axillary and is finally 
tilted mesally. The plica basalis now crosses the wing base obliquely 
from in front posteriorly and mesally (Fig. 133 B, hf). 

The final pull of the flexor muscle apparently is expended on the 
general wing base, for, in many insects, when the wing is fuUy flexed, 
the first axillary is revolved into a vertical plane on its hinge with the 
tergum, and the second axillary is thereby lifted, turned into a nearly 
longitudinal position, and brought close against the side of the back. 
A movement of the first and second axillaries, however, does not always 
accompany the ndng flexion, the essential changes in the basal region 
being the revolution of the third axillary and the folding along the line of 
the plica basalis at the bases of the median and cubital veins. 

As the posterior edge of the flexing wing comes against the side of the 
body, the jugal lobe (Fig. 133 A, Ju) is deflected and turned beneath 
the vannus along the line of the plica jugalis (B). If a plica vannalis 
(vf) is present, the remigial region {Rm) may be turned downward (C) 
during the flexion of the wing though many insects, such as the flies 
and bees, keep both the remigium and the vannus in a horizontal plane. 
If the vannus is large it also may be deflected beneath the remigium, 



THE WINGS 


239 


as in the hind wing of the cicada (Fig. 127 B) in which both the jugum 
and the vannus are turned downward against the side of the abdomen 
beneath the sloping remigium. In some Lepidoptera the flexed hind 
wing folds also along supplementary Mnes of plication in the rear part 
of the wing. 

The flexing of the wing becomes a still more complicated process 
if the vannal region is particularly enlarged, as in Plecoptera, Orthoptera, 
and Dermaptera. In most of the Orthoptera the vannus of each hind 



Fig. 134. — Wings of a grasshopper, Dissosteira Carolina. A, forerving. B, hind wing. 
C, position and plication of wings folded over the body as shown in transverse section. D, 
plications of flexed and folded right hind ■aing in section, more enlarged. 


wing is so greatly expanded (Fig. 134 B) that, when the wing is flexed, 
it must be plaited and folded together like a fan in order to give space 
for the rest of the wing. The folding and plaiting of the fully flexed 
wings of a grasshopper are shown at C and D of Fig. 134. The narrower 
forewings, or tegmina (C, TF 2 ), overlap each other to form a rooflike 
covering with steeply sloping sides completely enclosing the more dehcate 
hind wings (TFs) folded beneath them. The membrane of most of the 
vannal region of each hind vdng is deeply inflected between the primary 
vannal veins (D), and the secondary veins (o, b, c) he in the troughs 
of the folds. In most of the Orthoptera the vannal fold of the hind ving 
lies between the postcubitus and the first vannal vein; the fold usually 
contains a secondary vein, the vena di^ddens (Figs. 124 B, 134 B, D, vd). 


240 


PRINCIPLES OF INSECT MORPHOLOGY 


The hind wings of Dermaptera and Coleoptera when flexed are short- 
ened hy folds across the veins in order that they may be covered by the 
protective forewings, or elytra. The fanlike hind wings of the Dermap- 
tera consist principally of the expanded vannal regions. When flexed, 
the fans are plaited between the veins and then folded twice across the 
veins. In the Coleoptera the large jugum (Fig. 127 D, Ju) is folded in 
the usual manner beneath the vannus, and the transverse plications take 
place in the distal part of the wing. The transverse folding results 
automatically from the structure and flexibility of the veins. 

Extension of the Wings . — Extension involves a reversal of the move- 
ments of flexion. The flexor muscles must first relax. It is probable, 
then, that a contraction of the basalar muscles (Figs. 128 C, 129, E'), 
pulling on the humeral angle of the wing base, extends the wing directly 
in most insects, though the action of these muscles in this capacity is 
often difficult to demonstrate in a dead specimen. On the other hand, 
mth insects in which the second axillary sclerite is elevated on the outer 
edge of the upturned first axillary in the fully flexed wing, it is clear that 
the wing may be extended by the downward pull of the subalar muscles 
(E") on the second axillary, for a pressure on this sclerite from above 
at once restores all the axillary elements to a horizontal plane and thereby 
spreads the wing. Some insects may be seen to extend the wings deliber- 
ately before taking flight, but with most species flight is practically 
simultaneous with the wing expansion. 

6. INSECT FLIGHT 

An object self-moved through the air must be able to create a differ- 
ence in the density, or pressure, of the air on opposite sides of it; motion 
takes place toward the region of lowered pressure. Flight by any heavier 
than air animal or machine that does not depend upon rising columns 
of air for its support must have a mechanism capable not only of produc- 
ing horizontal motion but also of creating a lifting force sufficient to 
overcome the pull of gra\dty. Most flying machines are so constructed 
that the force of the propeller gives only a forward drive, the lifting force 
in horizontal flight being the result of decreased pressure above the wings 
created secondarily by the motion of the plane. The wings of insects, 
on the other hand, furnish directly not only the driving power but the 
lifting force as well; that is to say, the movement of the wings creates a 
region of lowered pressure both before and above the body of the insect. 

The possibilities of a motor mechanism for aerial locomotion can 
be judged bj’’ studying the air currents the motor will produce if it is 
itself held stationary. The nature of the air eurrents produced by the 
ving vibrations of insects, when the insects are secured by the body 
in such a manner that the wing movements will not be hindered, has been 



THE WINGS 


241 


studied by Demoll (1918). By means of a simple apparatus consisting 
of a frame with horizontal bars on which were suspended fine owl feathers, 
Demoll, by observing the deflection of the feathers when an insect with 
its wings in rapid vibration was brought near them, was able to determine 
the direction of the air currents created by the wing movements. 

Experimenting in this way with insects of different orders, Demoll 
found that the air currents drawn toward the stationary insect by the 
vibrating wings come from in front, from above, from the sides, and from 
below, while the currents given off are all thrown out to the rear (Fig. 
135). The strength of the currents, however, is not the same from all 
directions, as is indicated by the relative thickness of the arrows in the 
diagrams. The air is drawn toward the insect most strongly from before 
and above the anterior part of the body; the outgoing eurrents are 
strongest in a horizontal or slightly downward direction. Most of the 
oncoming currents, therefore, are turned to the rear in the neighborhood 
of the insect’s body and are condensed in a small region behind it. 



Fig. 135. — Diagram showing direction and relative strength of air currents produced by the 
vibrating wings of a stationary insect. {From Demoll, 1918.) 

If the insect is free to move, the mechanical effect of the vibrating 
wings on the air will be the same as when the insect is held stationary; 
but, instead of moving the air, or instead of mo^dng the air to the same 
extent as before, the greater part of the wing force will propel the insect 
through the air opposite the direction of the air currents created when the 
insect is secured. In terms of mechanics, the direction from which a 
current is drawn by a stationary object is the direction of lowered pres- 
sure, while the opposite is that of increased pressure. According to the 
observations of Demoll, therefore, when an insect launches itself into 
the air and sets up a vibration of its wings, there is at once created 
before it and above it a region of decreased pressure, and the convergence 
of all the currents behind produces here a region of greatly increased 
pressure. The lowered pressure above counteracts the weight of the 
insect; the increased pressure behind drives the insect forward into the 
low-pressure region in front. 

The driving force of the insect’s wing movements probably depends 
upon the angle at which the wng surfaces cut the air. Slow-flying 



242 


PRINCIPLES OF INSECT MORPHOLOGY 


insects with broad wings, such as the butterflies and grasshoppers, keep 
the wing surfaces almost horizontal and fly more in the manner of small 
birds with comparatively few strokes of the wings in any unit of time; 
some of the large swallowtail butterflies even soar for short distances 
with the wings held stationary. The more swiftly flying insects, how- 
ever, having narrow wings, turn the wing surface more nearly vertical 
vdth each stroke, whether up or down, and, as Ritter (1911) says, “the 
insect flies fastest when the downstroke approaches a vertical direction,” 
because the curve of the upstroke is drawn forward in the direction of 
flight. 

The speed of insect flight may be very high considering the small 
size of insects, but it varies greatly with different species. Demoll 
(1918) has computed the flying rate of various species from the time 
in which indi-\dduals traversed a room, going direct from the dark side 
to the light. The hawk moths (Sphingidae) he found are the swiftest 
flyers, making a speed up to 15 meters a second. A tabanid fly (Tabanus 
hovinus), however, is a close second, going at a rate of 14 meters. A 
dragonfly (Libellula depressa), doing ordinarily 4 meters a second, is 
capable of 6 to 10 meters in the same length of time. A house fly travels 
from 2 to 2.3 meters a second; a bumble bee (Bombus) from 3 to 5; the 
honey bee, unladen, has a speed of 3.7 meters a second, but when weighted 
with pollen it makes only 2.6 meters in the same unit of time. 

Insects appear to have no steering apparatus other than the wings 
themselves. Ordinary observation, as well as the experimental tests 
made by Stellwaag (1916) on the steering powers of insects, show that 
little or no compensatory movements of the body or legs are made during 
flight. Stellwaag showed that living insects impaled on pins turn 
themselves to the right or left by a differential action of the wings 
when the latter are rapidly vibrating with the movements of flight. 
The muscles concerned in the differential, or steering, action of the wings 
must be the lateral muscles of the alar segments, which are those of the 
basalar and subalar sclerites (Fig. 129, E', B"), since these muscles 
alone have specific connections with the wings. The longitudinal and 
vertical muscles of the wing-bearing segments, though potent effectors 
of ving movements, can not unequally distribute their influence between 
the two sides of the segment. 

Not only can most insects guide their course adroitly in forward 
flight, but many of them are able to fly directly backward or side'wise 
without altering the position of the body. The dragonflies are par- 
ticularly adept in these modes of flight, but many of the smaller insects, 
such as the flies and bees, are quite equal to the dragonflies in their 
ability to dart suddenly to one side or rearward, while the head still 
points in the direction of the arrested forward flight. Reversed and 



THE WINGS 


243 


lateral flying is probably controlled also by the lateral muscles of the 
flight mechanism, but it is remarkable that organs so evidently fashioned 
for forward flight, as are the wings of insects, can function efficiently 
for producing motion in other directions. 

Still another feat that many insects perform on the wing with apparent 
ease is hovering. Presumably, in maintaining one position in the air, 
the wings are vibrated approximately in a horizontal plane, thus creating 
a region of decreased air pressure above the body of the insect, but none 
before it. The rate of the wing movements then must be just sufficient 
to create a balance with the force of gravity. 

GLOSSARY OF TERMS APPLIED TO THE WINGS 

Alula, or Calypter. — A pair of membranous lobes at the posterior angle of the 
wing base, particularly developed in some Diptera. 

Anal Fold («/). — See plica vannalis. 

Anal Veins (A). — All the veins between the cubitus and the jugal region, includ- 
ing, according to the Comstock-Needham system, the veins here called postcubitus and 
vannals. 

Arcuate Vein (1/). — See vena arcuata. 

AxiUary Cord {AxC). — The thickened, corrugated posterior edge of the articular 
membrane of the wing base, continuous with the posterior marginal fold of the 
alinotum. 

Axillary Plate {AxP). — The posterior sclerite of the wing base in Odonata, sup- 
porting the subcostal, radial, medial, cubital, and vannal veins. 

Axillary Region {Ax). — The region of the wing base containing the axillary 
sclerites. 

Axillary Sclerites. — The sclerites of the axillary region in the wing-flexing insects, 
partly differentiated in Ephemerida, represented by the axillary plate in Odonata. 

Basal Fold (6/). — See plica hasalis. 

Cells. — The areas of the wing membrane between the veins and cross-veins. 

Costa (C). — The usual first vein of the wing, typically marginal, connected 
basally with the humeral plate. 

Cross-veins. — Short veins between the lengthwise veins and their branches; 
numerous in net-veined wings, in others generally few and located in definite positions. 

Cubitus (Cm). — ^The usual fifth vein of the wing. 

First Axillary (lAx).- — -The anterior hinge plate of the wing base, associated with 
the base of the subcostal vein. {Vordere Tergalgelenkplatte.) 

First Median Plate (m). — A small sclerite of variable shape lying in the angle 
between the second axillary and the distal arm of the third axillary at the base of the 
mediocubital field; accessory to the third axillary in function, and usually attached 
to it. 

Fourth Axillary {AAx). — A posterior hinge plate of the wing base present in some 
insects, intervening between the third axillary and the posterior wing process of the 
tergum. {Hintere Tergalgelenkplatte.) 

Frenulum. — The spine or group of bristles arising on the humeral angle of the hind 
wing of most moths, projecting beneath the forewing, and often held here in a frenulum 
hook. 

Humeral Plate {HP). — The anterior preaxillary sclerite of the wing base support- 
ing the costal vein; very large in Odonata. 



244 


PRINCIPLES OF INSECT MORPHOLOGY 


Jugal Region {.Ju). — A posterior basal lobe or area of the wing set off from the 
vannal region by the plica jugalis, containing the vena arcuata and vena cardinalis 
when these veins are present. 

Media (ilf). — The usual fourth vein of the wing; its base, when not united with 
radius, associated with the median plates of the wing base along the fold of the plica 
basalis. 

Median Plates. — See first median plate and second median plate. 

Paranotal Lobes (pnZ). — ^Lateral lobes of the pronotum in certain fossil insects, and 
theoretical lobes of the mesonotum and metanotum supposed to be the precursors of 
the wings. 

Plica basalis {hf). — The basal fold of the wing, or line of flection between the base 
of the mediocubital fleld and the axillary region, forming a prominent convex fold in 
the flexed wing extending between the median plates from the articulation of radius 
with the second axillary to the articulation of the vannal veins with the third axillary. 

Plica jugalis (jif). — The jugal fold of the wing of some insects, or radial line of 
folding setting o5 the jugal region from the vannal region. {Axillary furrow, plica 
ano-jugalis.) 

Plica vannalis (vf). — The vannal fold of the wing, or radial line of folding usuallj’ 
between the cubital field and the first vannal vein, but somewhat variable in position 
{Anal furrow, plica analis.) 

Postcubitus {Pcu). — The usual sixth vein of the wing, represented by an inde- 
pendent trachea in most nymphal wings, associated basaUy with the cubitus in the 
adult. {First anal of Comstock and Needham in most cases.) 

Precosta (Pc). — A small first vein of the wing in certain fossil insects. 

Pteralia. — The articular sclerites of the wing base, including the humeral plate 
and the axillary plate or axillary sclerites. 

Radius {R). — The third vein of the wing; its base flexibly attached to the second 
axillary. 

Remigial Region, or Remigium {Rm). — ^The wing area anterior to the vannal fold, 
containing the costal, subcostal, radial, medial, cubital, and postcubital veins. {Pre- 
anal region, preclavus.) 

Second Axillary (flAx). — The pivotal plate of the wing base resting on the pleural 
wing process, connected with the base of the radial vein. 

Second Median Plate {m'). — A variable sclerotization at the base of the medio- 
cubital field, folding convexly on the outer edge of the first median plate along the 
plica basalis; often absent, or represented by the united bases of the medial and cubital 
veins. 

Subcosta {Sc). — ^The usual second vein of the wing, associated basally with the 
anterior end of the first axillary sclerite. 

Tegula {Tg). — A large, scale-like lobe overlapping the base of the forewing in some 
insects; usually represented by a small setigerous pad or lobe {tg) at the anterior root 
of the wing base. 

Third Axillary (3Ax). — ^The flexor sclerite of the wing base; the sclerite on which 
the flexor muscle is inserted. 

Vannal Region, or Vannus {Vn). — ^The wing area containing the vannal veins, or 
veins directlj’’ associated with the third axillary; when large, usually separated from 
the remigium by the plica vannalis; often forming an expanded fanlike area of the 
wing. 

Vannal Veins (IT, 2V, etc.). — The veins associated at their bases with the third 
axillary sclerite, and occupying the vannal region of the wing. (The “anal” veins 
except the first, or postcubitus.) 



THE WINGS 


245 


Veins. — The tubular thickenings of the wings springing from the wing base and 
branching distally. 

Vena arcuata (1/). — The first jugal vein. 

Vena cardinalis (2/). — The second jugal vein, usually appearing as a basal 
branch of the vena arcuata. 

Vena dividens (vd ). — A secondary vein present in some Orthoptera lying in the 
fold between the remigium and vannus. 

Wing Base. — The proximal part of the wing between the bases of the veins and the 
body, containing the humeral and axillary sclerites. 

Wing Regions. — The principal areas of the wings differentiated in the wing-flexing 
insects, and often separated by distinct lines of folding, including the axillary, remigial, 
vannal, and jugal regions. 



CHAPTER XI 


THE ABDOMEN 

The third division of the insect trunk, the abdomen, differs character- 
istically from the head and the thorax by its simplicity of structure and 
general lack of segmented appendages. The union with the thorax 
may be broad or constricted, but, except in the aculeate Hymenoptera, 
there is seldom any question as to the line of separation between the 
thoracic and abdominal regions of the body. The abdomen varies much 
in form in different insects. Its segments usually remain distinct, 
though some of the posterior segments are commonly reduced or absent. 



Fig. 136. — E.’.amples of the presence of twelve segments in the hexapod abdomen. A, 
end of abdomen of embryo of Grylloialpa. (From Heymons, 1895.) B, terminal segments 
of abdomen of an adult proturan, Acerenlultis confinis. (From Berlese, 1910.) 

In certain aberrent species, however, the entire abdomen may be greatly 
reduced in size. 

The usual number of segments in the abdomen of adult insects is 10 or 
11, and from embryological evidence it appears that the primitive 
number was no greater than 12. Twelve segments are well developed 
in adult Protura (Fig. 136 B), and the same number occurs in embryos 
of certain generalized insects (A), but in postembryonic stages possible 
remnants of a twelfth segment are rare. The twelfth segment of the 
Hexapoda appears to be the periproct, that is, the primitive endpiece 
of the body anterior to which the true somites are formed. In the 
Protura the tenth and eleventh segments are said to be differentiated 
during postembryonic development; but in all the true Insecta the 
definitive segmentation of the body is established before hatching. 

A reduction in the number of abdominal segments is the rule in both 
immature and adult insects generally. Eleven segments are distinct in 

246 


THB ABDOMEN 


247 


many of the more generalized insects, but in the higher orders not more 
than 10 segments are usually present, and sometimes only 9 are distinct. 
In the Collembola the number is reduced to six and the limits of some of 
these are obscured in certain forms. Generally reduction takes place at 
the posterior end of the body, but in many of the higher insects there is a 
tendency toward elimination of the first abdominal segment. 

In general the abdomen serves as a container of the principal viscera 
of the insect and is the chief part of the body that produces movements 
of respiration. On the ventral surface of its posterior part are situated 
the apertures of the genital ducts, with which are associated the organs of 
copulation and oviposition; the alimentary canal opens at the end of its 
terminal segment. The median female genital aperture varies in position ; 
in a few insects it is located just behind the seventh abdominal sternum, 
in others it is on or behind the eighth sternum, and in still others it is on or 
behind the venter of the ninth segment. The male aperture appears to be 
always on the posterior part of the ninth segment, except in CoUembola, 
in which the gonopore in each sex is between the fifth and sixth segments. 

For convenience of study the segments of the abdomen may be 
grouped into pregenital, or visceral, segments, genital segments, and 
postgenital segments. The genital segments are primarily the eighth 
and the ninth in the female, and the ninth in the male, since it is the 
appendages or other outgrowths of these segments that form the principal 
parts of the external genitalia. One or more segments preceding and 
following the primary genital segments, however, are frequently involved 
in the genital modifications of the abdomen, and it is often found more 
expedient to divide the abdomen accordingly into a preabdomen and a 
postdbdomen. In the higher Diptera, for example, the first five segments 
form a distinct preabdomen, while the remaining segments are more or 
less modified as a part of the genital apparatus, including the long 
telescopic “ovipositor” of the female (Fig. 312 B, C). 

Notwithstanding the simplicity of appearance in the structure of the 
abdomen and the retention of individuality of its segments, the abdomen 
is in many respects a highly modified and specialized region of the body. 
Though its sclerotized areas have usually the form of simple segmental 
plates, the sterna at least are evidently composite structures; and, while 
segmental appendages are characteristically absent, such rudiments of 
them as do persist raise questions in morphology that are difficult to 
settle. 

1. GENERAL STRUCTURE OF THE ABDOMINAL SEGMENTS 

The abdominal segments of adult insects for the most part are typical 
secondary segments, the functional conjunctivae being the membranous 
posterior parts of the primitive somites. The primary intersegmentai 



-248 


PRINCIPLES OF INSECT MORPHOLOGY 


folds usually form internally submarginal antecostae on the definitive 
tergal and sternal plates, to which the longitudinal muscles are attached, 
and they are marked externally by corresponding antecostal sutures. 
The terga and sterna regularly overlap posteriorly (Fig. 37). In soft- 
bodied larval insects the abdominal segmentation is more nearly of the 
primary type, though in holometabolous. larvae there is a tendency for the 
longitudinal muscles to become separated into groups of fibers that do not 
all have intersegmental attachments. 

The generalized form of an abdonnnal segment is approximately 
retained in larval insects that preserve rudiments of the abdominal 
appendages. In an ephemerid larva, for example (Fig. 150 A, B), each 
gill-bearing segment is distinctly divided into a dorsum and a venter by 
large lateroventral lobes (Cxpd) supporting the gills, which evidently 



Fig. 137. — Sclerotization of the abdomen. A-E, examples of variation in the 
abdominal sclerotization above and below the dorso-ploural line (a-a). F, typical second- 
ary segmentation of the abdomen. G, inner view of consecutive tergal plates, with muscle 
attachments. 


represent the bases of abdominal appendages. Generally, however, the 
limb bases are more or less united with the venter, and in the adult insect 
the sternal plate of each segment is usually a continuous sclerotization of 
the ventral and pleural regions. 

The Abdominal Sclerotization. — Thesclerotized parts of the abdominal 
integument usually take the form of dorsal and ventral segmental plates, 
separated by membranous areas on the sides (Fig. 139 A). In certain 
larval and adult insects, however, there are four distinct series of abdom- 
inal plates, namely, dorsal tergal sclerites, lateral pleural sclerites, and 
ventral sternal sclerites. Thus in some of the Thysanura (Fig. 138 A) 
each abdominal segment presents a broad tergal plate (T) above, a small 
median sternal plate (S(n) below, and, flanking the latter, a pair of large 
pleural plates (Cxpd). The pleural plates of the Thysanura, it is gen- 


THE ABDOMEN 


249 


erally conceded, represent the bases of abdominal limbs. A generalized 
abdominal segment, therefore, we may assume, had a tergum occupying at 
least the major part of the dorsum (Fig. 137 A, T), a pleuron {PI), or a 
group of pleurites, on each side situated in the area of the limb base (P), 
and a sternum {Stn) in the venter. In modern insects, however, the 
relation of the definitive abdominal sclerotization to the morphological 
regions of the body is highly variable, and the numerous anatomical 
inconsistencies that arise create many difficulties in nomenclature. 

In the usual condition found in adult and nymphal insects the 
primitive pleura and sternum of each segment (Fig. 137 A, PI, Stn) are 
united in a continuously sclerotized definitive sternal -plate (B, S) opposed 
to the tergum. If the tergal sclerotization extends domiward on the 
sides of the dorsum so far as to include the spiracular areas, the spiracles 



Fig. 138. — Pleurostornal plates of the abdomen. A, under surface of abdominal 
segment of Nesomachilis, showing true sternum (Stn) and plates of limb bases (Cxpd). B, 
abdominal sternum of Heterojapyx, with limb bases united with sternum, and a sternal 
apotome (Apt) separated from the latter. C, definitive sternal plate, or coxosternum, of 
ninth abdominal segment of male termite, Termopsis. 

will be enclosed in the lateral parts of the tergum (C). In many cases, 
however, the dorsum contains lateral tergal sclerites, or laterotergites 
(D, Itg), quite distinct from the principal median tergite {mig). The 
laterotergites often contain the spiracles, but the spiracles may be located 
in the membrane above or below the laterotergites. With some insects, 
again, the spiracles occur in lateral parts of the ventral plates (E), and in 
such cases it is evident that the definitive sterna are continuous sclerotiza- 
tions of the primary sternal, pleural, and laterotergal areas. Finally, 
as in the larvae of Plecoptera and in the male genital segment of many 
adult insects, the tergal, pleural, and sternal plates may become con- 
fluent in a continuously sclerotized annulus. 

The Abdominal Terga. — The dorsal sclerotization of an abdominal 
segment usually has the form characteristic of a secondary segmental 
plate (Fig. 137 F). A typical abdominal tergum (T), therefore, presents 
anteriorly a marginal or submarginal ridge, the antecosta (F, G, Ac), on 
which the principal longitudinal muscles usually have their attachments. 
The antecostal suture (F, acs) is generally but faintly marked, and the 
precostal acrotergite {atg) varies from a scarcely perceptible marginal rim 
to a fairly wide flange extending anterior to the muscle attachments (G). 




250 


PRINCIPLES OF INSECT MORPHOLOGY 


In some cases, however, the antecosta and acrotergite are lost and the 
muscles attach simply on the anterior edge of the tergum. Apodemal 
arms are sometimes developed from the anterior margins of the abdominal 
terga, which give effectiveness particularly to protractor muscles inserted 
upon them. Behind the tergum is the conjunctival membrane (F, Mh), 
and the abdominal terga regularly overlap posteriorly, except where 
successive segmental plates are united. 

In many insects, particularly in larval forms, the dorsal sclerotization 
of the abdomen may be broken up into groups of segmental tergites. 
In simple cases we may distinguish in each segment a median tergite 
(Fig. 139 B,r?z(g) and one or more laterotergites (Z(g) ; but often the sclerotiz- 
ation of the median area is again subdivided into smaller sclerites. The 
lower limit of the dorsum must be determined by discovering, where 
possible, the position of the dorso-pleural line (o-a), which is often marked 
by a lateral groove extending into the thorax above the subcoxal pleurites 
{Sex) . 

Abdominal Pleurites . — Strictly defined, an abdominal plate properly 
called a pleural sclerite is a sclerotization in the region of the abdomen 
corresponding to that of the subcoxal pleural plates of the thorax. An 
abdominal pleurite, therefore, is presumably a derivative of the primitive 
basis of an abdominal appendage. The stylus-bearing plates of the 
abdomen of some Thysanura (Fig. 138 A), the gill-bearing lobes of 
ephemerid larvae (Fig. 150), the basal plates of the ovipositor (Fig. 35 C), 
or the lateral sclerites in the abdomen of many holometabolous larvae 
(Fig. 139 B, fl), lying between the dorso-pleural and pleuro-ventral 
grooves, are examples of abdominal plates that may very evidently be 
referred to the true pleural region. But, again, it is undoubtedly true 
in many cases that small sclerites occurring in the pleural region of the 
abdomen are secondary sclerotizations, or lateral subdivisions of the 
definitive pleurosterna, and thus cannot be supposed to represent literally 
the bases of abdominal limbs. Such sclerites are sometimes designated 
later osternites, though they are pleurites in the sense that they lie in the 
pleural region. The term “pleurite,” however, should not be given to 
laterotergites or sclerites that lie clearly above the dorso-pleural line 
(Fig. 139 B, Itg), such as those often called “ epipleurites ” in descriptive 
entomology. In many species of lepidopterous larvae the serial identity 
of the thoracic and abdominal areas or their sclerites is shown by corre- 
sponding setae or groups of setae located on them (Fig. 153 A). 

The Abdominal Sterna . — The definitive sternal plates of the abdomen 
of adult insects are in general similar to the tergal plates, each including a 
primary intersegmental area in its anterior part (Fig. 137 F). The 
antecostae (Ac) may be coincident with the anterior margins of the plates 
or preceded by distinct acrosternal flanges. The sterna of most adult 



THE ABDOMEN 251 

insects, however, as we have seen, are evidently composite plates (Fig. 
137 B, S), each formed by a union of the primary sternum (A, Stn) with 
the regions of the primitive limb bases {PI) . The frequent occurrence of 
styli on the ninth abdominal sternum of the male in more generalized 
pterygote insects (Fig. 138 C) attests the triple composition of the sternal 
plate, since in some of the apterygote insects the stylus-bearing plates are 
either only partially fused with the sternum (B, Cxpd) or entirely free 
from it (A). A definitive sternal plate that includes the areas of the limb 



rntg- Ifcg* 



Strig / 

C _ 

Fig. 139. — Examples of abdominal sclerotization Tvith reference to the dorso-pleural 
line (a-a) and the pleuro-ventral line (6-6). A, metathorax and abdomen of adult male 
Gryllus. B, metathorax and abdomen of larva of Calosoma. Itg, laterotergites ; mtg, 
medio tergite; pi, pleurites; stn, sternites. 


bases is morphologically a coxosternum, or pleurosternum. The limb base 
elements of such a sternum (commonly called “coxites”) are coxosternites, 
or pleurosternites. 

The ventral sclerotization of the abdomen, as that of the dorsum, 
may be broken up into a group of sternites, as in various holometabolous 
larvae (Fig. 139 B, sin). In the Japygidae a short anterior subditdsion 
of each abdominal sternum is separated by a membranous fold from the 
rest of the plate, forming a distinct sternal apotome (Fig. 138 B, Apt). 

Characteristics of the Abdominal Segments. — Adult insects having 
well-developed organs of copulation and egg laying usually show a dis- 
tinct differentiation in the structure of the segments of the visceral, 
genital, and postgenital abdominal regions. The modifications affect 
principally the genital segments, which are structurally adapted to their 
special functions, and the postgenital segments, which generally suffer 





252 


PRINCIPLES OF INSECT MORPHOLOGY 


reduction in proportion to the h 3 rperdevelopment of the genital segments. 

The Visceral Segments. — The segments of the visceral region of the 
abdomen are usually of simple structure and differ but little from one 
another. In adult pterygote insects they never bear appendicular organs. 
The first segment is either broadly joined to the thorax or separated from 
it by a constriction. In winged insects the antecosta of the first abdom- 
inal tergum bears the third pair of phragmatal lobes (Fig. 97 D, Wh), 
and the acrotergite is usually much enlarged, forming the postnotal plate 
of the metathorax (PNs), which, together with the base of the phragma, 
is frequently detached from the rest of the first abdominal tergum and 
becomes virtually a part of the metathorax. The rest of the first seg- 
ment is often reduced or fused with the second, and the sternal sclerotiza- 
tion is sometimes obliterated. In the clistogastrous Hymenoptera the 
entire first abdominal segment is so intimately united with the metathorax 
that it forms anatomically a part of the thorax, termed the propodeum. 
In these insects the constriction between the apparent thoracic and 
abdominal sections of the body occurs between the first and second seg- 
ments of the abdomen. In some of the ants the second segment is small 
and a second constriction occurs between it and the third segment. 

The Genital Segments. — In some of the simpler insects there is no 
modification of the segments associated with the genital apertures to 
distinguish them as genital segments; but usually the ninth segment in 
the male, and the eighth and ninth in the female show some structural 
adaptation to the genital functions. 

Modifications of the eighth segment occur principally in female 
insects having a well-developed OAupositor, since the first valvulae of 
the ovipositor are developed from this segment. The valvulae are borne 
directly by small pleural plates, the first valvifers (Fig. 314 A, B, IVlf), 
which correspond to the stylus-bearing plates of the Thysanura, though 
styli themselves are absent from the eighth segment in all pterygote 
insects. The sternum of the eighth segment may be a simple plate 
resembling the sterna preceding it, but often it is enlarged and produced 
posteriorly beneath the base of the ovipositor. In such cases it forms 
the female subgenital plate. On the other hand, the eighth sternum is 
sometimes reduced, and in some insects it is practically obliterated. This 
condition is usually accompanied by an enlargement of the seventh 
sternum, which then becomes the subgenital plate. 

The second genital segment usually has less of the typical segmental 
form than does the first. It is the segment of the second and third 
vahmlae of the ovipositor in the female. These vahmlae are borne by 
pleural scleritcs of the ninth segment (Fig. 314 A, B, 2Vlf), the second 
valvifers (eommonly called “coxites”)* which correspond to the valvifers 
of the eighth segment. Rudimentary styli occur on the second valvifers 



THE ABDOMEN 


253 


in nymphal and adult forms of some of the lower Pterygota (E, Sty), but 
generally they are absent from the ninth segment of the female, except in 
Thysanura. The venter of the ninth segment in the female is usually 
inconspicuous, and, where an ovipositor is present, it is reduced to a 
narrow membranous space between the vahnilae, but it may contain 
intervalvular sclerotic remnants of the ninth sternum. 

In the male the ninth segment retains a generalized structure in the 
Thysanura, but in pterygote insects it is subject to many modifications 
and takes on a great variety of forms. The dorsal and ventral areas are 
usually sclerotized and form definite tergal and sternal arcs of the seg- 
ment. In some insects the bases of the male gonopods are distinct plates 
having a normal pleural position on the sides of the ninth segment between 
the tergum and the sternum, and in such cases they usually bear movable 
lobes, serving generally as claspers, which apparently represent the styli 
of generalized insects. The gonopod bases, however, may be united with 
the sternum, and the resulting coxosternal plate then carries the styli, 
if the styli are preserved, which retain a typical styliform shape in some 
Orthoptera and Isoptera. On the other hand, the basal plates of the 
gonopods may be displaced posteriorly as free lobes bearing the claspers, 
or, again, they become fused with both the tergum and the sternum in a 
continuous segmental annulus. 

The modifications of the genital segments and the structure of the 
organs of copulation and egg la3dng will be more fully described in Chap. 
XIX. 

The Tenth Segment . — The tenth segment is present in the abdomen of 
nearly all insects, but its limits are often difficult to determine because 
of the frequent union between the tenth and eleventh segments. When 
only one postgenital segment is retained, as in the majority of holometab- 
olous insects, both larval and adult, this segment is presumably the tenth. 
It sometimes bears a pair of appendicular processes, such as the socii of 
adult Trichoptera and Lepidoptera, the cercuslike appendages of adult 
Tenthredinidae, and the postpedes of larval Neuroptera, Trichoptera, 
Lepidoptera, and Tenthredinidae. The tenth segment appendages may be 
termed collectively the pygopods, since the tenth segment is the pygidial, 
or “rump,” segment. The tenth segment sometimes bears lobes or 
processes that clearly have no relation to appendages. 

When two postgenital segments are present, as in many of the more 
generalized insects, the tenth segment is frequently reduced and more 
or less united with the ninth or the eleventh segment. In none of the 
exopterygote insects does it have appendages in postembryonic stages, 
though rudiments of limbs may be present on it in the embryo (Fig. 136 
A, XApd). The tenth segment, accompanied by the eleventh, occurs as 
a complete and independent annulus among the Thysanura (Fig. 140 



254 


PRINCIPLES OF INSECT MORPHOLOGY 


A, X), Odonata (C), Ephemerida (D), Dermaptera (I), Homoptera (L), 
and in females of Panorpidae. In the Ephemerida its tergal plate (D, 
XT) is produced posteriorly in a small truncate lobe between the bases 
of the cerci and thus resembles the supra-anal plate of some other insects 
formed of the eleventh tergum, but in the ephemerid the dorsal part of 
the eleventh segment, or true epiproct, lies beneath the lobe of the tenth 



Fig. 140. — Postgenital segments of the abdomen. A, B, Nesomachilis maoricus. C, 
Plalhcmis lydia, adult male, ventral ^^e^v. D, ephemerid, adult male. E, perlid larva. F, 
embiid. G, Gryllus assimilis. H, Blalla orientalis, ventral \’iew. I, Anisolahis mari- 
lima, female. J, Blalla oricnlalis, dorsal view, segments separated. K, Dissosleira 
Carolina, female. L, Magicicada scptendccim, male. An, anus; Ccr, cercus; cf, caudal 
filament; cxpd, base of cercus (coxopoditc) ; Eppt, epiproct; Ovp, ovipositor; Papt, paraproct; 
papll, lobe of paraproct; sal, supra-anal lobe; xmds, muscles of tenth segment. 


tergum and carries the median caudal filament (cf). Likewise, in the 
Plccoptera (E), Embiidae (F), and Blattidae (J), the tenth tergum (XT) 
is the terminal dorsal plate of the abdomen; the epiproct in these insects 
is reduced to a supra-anal pad or membrane beneath the end of the tenth 
tergum. In the Orthoptera the ventral part of the tenth segment is 
mostlj' membranous and usually does not appear in the adult as a defi- 
nitely defined segmental region, though it sometimes contains a small 
sternal sclerotization. The tergum of the segment is generally a distinct 
plate, as in Acrididae (K), but sometimes it is more or less united with the 


THE ABDOMEN 


255 


epiproct to form a composite supra-anal plate, though the division 
between the two parts may remain quite evident, as in Gryllus (G). 

The Eleventh Segment. — The eleventh segment of the abdomen repre- 
sents the last true somite of the body. It is present in the embryos of 
lower insects as a well-developed metamere bearing the rudiments of the 
terminal pair of appendages, which are the cerci of the imago (Fig. 136 A, 
XI). In adult Protura (B) it is a normal annulus with tergal and sternal 
plates; but in all the true Insecta the eleventh segment is more or less 
reduced, and its individuality is often lost by union with the tenth seg- 
ment. In most of the Holometabola it is suppressed entirely, and the 
body ends with the tenth segment. 

To generalize on the structure of the eleventh segment, we may say 
that, when present, it forms a conical endpiece of the body, bearing the 
cerci laterally and the anus at its apex; its dorsal surface is covered by a 
triangular or shield-shaped tergal plate, the epiproct (Fig. 140 K, Eppt), 
and its ventrolateral parts form two lobes, the paraprocts (Papt). The 
ventral margins of the paraprocts are usually connected basaUy by a 
median membranous area (C, H), and the posterior margin of the latter 
is sometimes produced in a small subanal lobe, or hypoproct. Occasion- 
ally the paraprocts bear terminal lobes, such as the small, soft, apical 
parts in some adult Odonata (C, paptl), the stylus-like processes of the 
paraprocts in tridactylid Orthoptera, or the broad, tracheated plates 
forming the lateral gills of zygopterous odonate larvae (Fig. 141 C, paptl). 

The cerci are implanted typically in membranous areas between the 
bases of the epiproct and the paraprocts behind the tenth tergum (Fig. 
140 K, Cer). Though they are generally closely associated with the 
tenth segment, embryologists mostly agree that they arise in the embryo 
as limb rudiments on the eleventh segment (Fig. 136 A, XIApd). Their 
connection with the tenth segment becomes more pronounced with the 
reduction of the eleventh segment or its union with the tenth. In Cam- 
podeidae and Japygidae the abdominal segments beyond the tenth 
are obliterated, but the cerci are retained and are necessarily borne 
directly on the end of the tenth segment. In Machilidae each cercus is 
supported on a large pleural lobe of the eleventh segment (Fig. 140 A, 
Cxpd), and in many of the more generalized Pterygota the appendage 
has a small, usually imperfect basal segment (G, Cxpd). The shaft of 
the cercus is sometimes distinctly divided into segmentlike sections 
(D, F), but it never contains muscles. Most of the muscles that move 
the cerci, which are inserted on or near the cereal bases, take their 
origins on the tenth tergum and are probably muscles of the tenth seg- 
ment (A, xmcls). In some insects one muscle of each cercus arises on the 
epiproct, but the cerci never have muscles from the paraprocts. The 
latter, therefore, do not have the relation of limb bases to the cerci. 



256 


PRINCIPLES OF INSECT MORPHOLOGY 


The cerci of insects apparently correspond to the uropods of malacostra- 
can Crustacea. They are usually simple processes, conical or filamentous 
in form, and of a sensory function, but sometimes they are modified to 
serve as clasping organs. 

The generalized structure of the eleventh segment is perhaps most 
fully retained in the Machilidae. In Nesomachilis, for example (Fig. 
140 A), the eleventh segment, though normally concealed within the tenth, 
has the form of a complete ring with distinct tergal and sternal regions 
separated on the sides by the large lateral lobes (Cxpd) bearing the cerci 
(Cer). The tergal region (XIT) is produced into the median caudal fila- 
ment (c/) . The ventral region presents anteriorly a narrow sternal bridge 
(B, XlStn) between the lateral cercus-bearing lobes, and posteriorly a 
pair of broad paraprocts (Papt) at the sides of the anus. 

Among the lower Pterygota, the parts of the eleventh segment are 
entirely distinct from the tenth segment in Odonata. In an adult 
dragonfly (Fig. 140 C) the epiproct (Eppt) is a large free median lobe 
tapering to a truncate point. The cerci arise laterad of the epiproct 
and are broadly hinged to the posterior margin of the tenth tergum. The 
paraprocts {Papt) are wide triangular ventral lobes at the sides of the 
anus; each contains a large basal plate and terminates in a small fleshy 
process (papil). In the larvae of anisopterous Odonata the epiproct and 
paraprocts form the three tapering valvular processes that close the large 
anal opening (Fig. 141 A, B). In zygopterous larvae each lobe of the 
eleventh segment bears a gill plate (C), the median gill (cf) being a process 
of the epiproct, and the lateral gills {papil) processes of the paraprocts. 
The small cerci {Cer) arise in the usual position. In the Ephemerida 
(Fig. 140 D) the reduced epiproct bearing the caudal filament (c/) is 
concealed beneath the overhanging median lobe of the tenth tergum; 
and in the Plecoptera (E) and Embiidae (F) the epiproct is reduced 
to a supra-anal pad adnate to the ventral surface of the tenth tergum. 
In Dermaptera (I), however, the epiproct {Eppt) is a distinct plate 
between the bases of the cerci, movably hinged to the posterior margin of 
the tenth tergum. In most orthopteroid insects the eleventh segment is 
distinct, though often closely united with the tenth (K), and the epiproct 
may be fused with the tenth tergum (G); but in Blattidae (H, J) the 
epiproct is practically obliterated except for a membranous fold beneath 
the tenth tergum on wliich the muscles of the paraprocts (H, Papt) are 
attached. 

The Twelfth Segment. — ^The primitive terminal segment of the arthro- 
pod trunk is the periproct, or endpiece of the body containing the anus, 
anterior to which the true appendage-bearing somites are formed. In the 
malacostracan Crustacea the periproct forms the telson, typically a 
broad terminal lobe of the abdomen having the anus situated in the basal 



THE ABDOMEN 


257 


part of its ventral surface. The periproct appears to be represented 
in the embryos of some insects by a terminal twelfth segment of the 
abdomen (Fig. 136 A, Prpi), which never has appendages; but among 
adult hexapods a twelfth abdominal segment with tergal and sternal plates 
occurs only in the Protura (B). In most insects no trace of a twelfth 
segment is to be found, and the periproct must be supposed to be repre- 
sented, if at all, only by the circumanal membrane at the end of the elev- 
enth segment. 

The best example of the possible retention of a twelfth abdominal 
segment in postembryonic stages of insects is furnished by the larvae of 



showinR possible rudiment of twelfth segment, or periproct (Prpt). B, some, Intcrnl view, 
with parts in usual position. C, Archilcstcs grandis larva. 

anisopterous Odonata, in which the anus is contained in a small circular 
fold (Fig. 141 A, Prpt) ordinarily concealed between the bases of the 
cpiproct {Eppt) and the paraprocts {Papt). In the walls of this fold there 
is a small dorsal sclerite, or lamina supra-analis (sal), and two lateroven- 
tral sclerites, or laminae infra-anales {lal). These sclerites are lost in 
adult Odonata, but a small supra-anal lobe, apparently a remnant of the 
lamina supra-analis, projects from beneath the epiproct (Fig. 140 C, sal). 
A similar lobe occurs in larva of Ephemerida and in some adult 
Thysanura (B, sal). The supra-anal lobe of these insects, therefore, 
might be regarded as a dorsal remnant of the telson. 

2. THE ABDOMINAL MUSCULATURE 

The abdominal musculature of adult and larval insects in general 
conforms to a rather simple fundamental pattern, which is repeated with 
only minor variations in each of the pregenital segments; in the genital 
and postgenital segments the basic plan of musculature is more or less 
obscured by special modifications. In some of the Apterygota, however, 
and in larval forms of holometabolous insects the body musculature may 



258 


PRINCIPLES OF INSECT MORPHOLOGY 


be liigbly complex. Some writers have regarded the complex types of 
muscle arrangement as representing a primitive condition; but since these 
types have no conformity among one another, and since the musculature 
of holometabolous larvae shows in all orders a progressive evolution away 
from the simple adult type, it would seem that the latter must be more 
nearly representative of the muscle pattern of primitive insects. The 
abdominal musculature of adult insects is more elaborate than the body 
musculature of the thorax, but on the whole it is simpler than the thoracic 
musculature because of the absence of leg muscles. 

Since the muscles are derived from the walls of the embryonic coelomic 
sacs, or at least from the metameric divisions of the mesoderm, we may 
assume that the primitive somatic fibers of arthropods were all intra- 
segmental in arrangement (Fig. 35 A), as they are in the Annelida; but 
with the acquisition of secondary segmentation, consequent upon the 
development of sclerotic plates in the body wall, the longitudinal fibers 
become functionally intersegmental (B). The body of the animal can 
thus be shortened by a telescoping of its segments (C) brought about by 
contraction of the longitudinal muscles, and it can be compressed by 
contraction of the lateral dorsoventral muscles. The opposite move- 
ments may result either from the elasticity of the body wall or from 
pressure generated by contraction in one part of the body transmitted to 
another through the medium of the body liquid and the visceral organs; 
but in the abdomen of the higher arthropods protractor and dilator 
apparatus are developed in which certain muscles become antagonistic to 
the retractors and compressors. 

General Plan of the Abdominal Musculature. — The muscles of the 
insect abdomen may in general be classed in three groups, namely, 
dorsal muscles, ventral muscles, and lateral muscles. The dorsal muscles 
include longitudinal dorsals and transverse dorsals; the ventral muscles are 
similarly dmded into longitudinal venirals and transverse ventrals; the 
lateral muscles comprise lateral muscles of the body wall and spiracular 
muscles. Each of these sets of muscles is again often subdivided into two 
or more minor groups. The naming of the muscles according to this 
classification would, in a final analysis, lead to the compounding of terms 
of unwieldy length. Hence the writer (1931) has proposed a scheme for 
simplifjnng the nomenclature by limiting the terms “dorsal” and “ven- 
tral” to the longitudinal dorsal and ventral muscles only, and dividing the 
transverse muscles into dorsal and ventral sets. According to this plan 
the major groups of muscles are as follows: 

L Dorsal muscles (Fig. 142 A, d), the fibers of which are typically longitudinal 
and attached on the intersegmental folds or on the antecostae of successive terga. 

n. Ventral muscles (v), resembling the dorsal muscles in that their fibers are 
typicall.v longitudinal and attached on the intersegmental folds or on the antecostae of 
successive sterna. 



THE ABDOMEN 


259 


III. Lateral muscles (1), typically dorsoventral, and both intrasegmental (le) and 
intersegmental (li) in position. 

IV. Transverse muscles (C, t), lying internal to the longitudinals, including dorsal 
transverse muscles {Id) and ventral transverse muscles (tv). 

V. Spiracular muscles, generally not more than two connected with each spiracle, 
one an occlusor, the other a dilator. 

Each of the first three of these primary groups of muscles may undergo 
an endless diversification resulting from a multiplication of fibers in the 
group, a separation of the fibers into subgroups, or a rearrangement of the 
fibers brought about by changes in the points of attachment. 

With respect to the dorsal and ventral muscles the most general 
departure from the simple plan, in which the fibers all lie in a single 



Fia. 142. — Diagrams of abdominal musculature. A, B, simple types of musculature, 
right half of a segment, inner view. C, cross section of a segment. (For lettering see 
page 260.) 

plane against the body wall, consists of a differentiation of the fibers 
in each group into external muscles and internal muscles. Thus it is found 
in nearly all insects that the dorsal and ventral muscles comprise each two 
layers, there being, namely, internal dorsals [di) and external dorsals (de), 
and internal ventrals {vi) and external ventrals (ve). A second form of 
diversification affecting the same muscles consists of a more or less distinct 
grouping of the fibers into median and lateral sets. In most insects, 
therefore, we may distinguish four sets of dorsal fibers, and four sets of 
ventral fibers. The several resulting muscles or sets of fiber bundles then 
may be designated as follows : median and lateral internal dorsals (Fig. 143 
A, dim, dil), median and lateral external dorsals {dem, del), median and 
lateral internal ventrals {vim, vil), and median and lateral external ventrals 
{vem, vel). 

In some insects there is a longitudinal muscle or group of longitudinal 
fibers situated on the lateral part of the dorsum above tlie line of the 
spiracles, external to the upper ends of the internal lateral muscles. This 
muscle is sometimes called a “pleural” muscle, but since it evidently 
belongs to the dorsum it is more properly termed a varadorsed muscle 
(Figs. 142 B, C, 143 B, p). 



260 


PRINCIPLES OF INSECT MORPHOLOGY 


The lateral muscles are more subject to irregularities of position than 
are the dorsal and ventral muscles, but they likewise are often divided into 
internal laterals (Fig. 142, U) and external laterals (le). 

To express more concisely the major groups of abdominal muscles and 
their principal subdivisions, we may tabulate the several sets of fiber 
bundles enumerated above as follows: 

I. Musculi dorsales (d). 

1. M. dorsales interni (di)- 

a. M. dorsales interni mediales (dim). 

b. M. dorsales interni laterales (dil). 

2. M. dorsales extemi (de). 

a. M. dorsales extemi mediales (dem). 

b. M. dorsales extern! laterales (del). 

3. M. paratergales (p). 
n. Musculi ventrales (v). 

1. M. ventrales interni (vi). 

a. M. ventrales interni mediales (vim). 

b. M. ventrales interni laterales (vil). 

2. M. ventrales extern! (we). 

a. M. ventrales extemi mediales (vem). 

b. M. ventrales extemi laterales (vel). 
ni. Musculi laterales ( 1 ). 

1. M. laterales interni (li). 

2. M. laterales extemi (le). 
rv. Musculi transversales (t). 

1. M. transversi dorsales (id). 

2. M. transversi ventrales (Iv). 

V. Musculi spiraculorum 

1. M. occlusores spiraculorum (osp). 

2. M. dUatores spiraculomm (dlsp). 

It is often difficult to define individual muscles of the body wall of 
insects because the fiber bundles are not surrounded by a common 
sheath; but generally the muscles are distinct because of the grouping 
of the fibers and may be given individual names. For reference pur- 
poses, however, it will be found more practical to indicate individual 
muscles on drawings with Arabic numerals, since it is often difficult 
or impossible to identify corresponding muscles throughout the series 
of segments. Though in the visceral region of the abdomen the muscles 
may be segmentally repeated with fair regularity, the arrangement is 
usually so distorted in the genital and postgenital regions that the 
muscle homologies become very doubtful. 

We may now give a brief summary of the principal modifications in 
the arrangement of the muscles of the several principal muscle groups 
in the visceral segments of the abdomen. The musculature of the genital 
and postgenital segments requires a special study and vdll not be con- 
sidered here. 



THE ABDOMEN 


261 


The Dorsal Abdominal Muscles . — The muscles of the dorsum are 
composed primarily of longitudinal fibers of segmental length attached 
on the intersegmental folds (Fig. 142 A, d). In many larvae the principal 
dorsal fibers retain this primitive condition; but in insects having fully 
developed tergal plates the dorsal muscles become functionally inter- 
segmental because the folds on which they are attached become the ante- 
costae of the definitive terga (Fig. 143 C). Since the segmental plates 
are pulled forward by the contraction of the longitudinal muscles, the 
anterior end of a longitudinal abdominal muscle may be termed its origin, 
and the posterior end its insertion. 

The internal dorsals commonly retain their longitudinal position 
and their segmental length (Fig. 143 C, D, di ) ; but they undergo many 
departures from this generalized condition through becoming oblique 
or by a shift in their origins to the postcostal regions of the terga. The 
external dorsals, on the other hand, are seldom of segmental length; 
typically they are short muscles l 3 ring in the posterior parts of the seg- 
ments (C, de), and often they become strongly oblique, sometimes 
actually transverse, giving a movement of torsion to the segments they 
connect. Finally, their origins may become transposed to the posterior 
margins of the terga, in which case the external dorsals are reversed 
in position (D, de)] functionally they then become antagonistic to the 
internal dorsals {di) and act as abdominal protr actors, since their contrac- 
tion lengthens the abdomen by decreasing the overlap of the segments. 
In some cases the anterior ends of the protractors are attached on apode- 
mal arms of the anterior margins of the terga, thus increasing the effective- 
ness of the muscles. The dorsal muscles are often variously reduced, and 
some of the principal groups of fibers may be entirely suppressed. 

The paradorsal muscle (Figs. 142 B, C, 143 B, p) is not commonly 
present in adult insects, or, at least, its fibers are not generally separated 
from those of the other lateral dorsal muscles. It is well shown in the 
Acrididae as a distinct muscle (Fig. 144 A, 169), and it is a characteristic 
feature of the musculature of some larval insects. 

The Ventral Abdominal Muscles . — The ventral muscles of the abdomen 
undergo an evolution parallel in most respects to that of the dorsal 
muscles. The fibers of the internal layer are typically intersegmental 
wherever complete sternal plates are present and serve as retractors 
of the ventral arcs of the segments. The external ventrals are usually 
short and take their origins on the posterior parts of the sterna. Fre- 
quently they become sternal 'protractors by a reversal in position, and 
commonly their anterior ends are then carried forward on anterior 
apodemal arms of the sterna (Fig. 144 A, 174). The ventral muscles, as 
the dorsal muscles, however, are sometimes reduced, and one or more of 
the principal groups may be lost. 



262 


PRINCIPLES OF INSECT MORPHOLOGY 


The Lateral Abdominal Muscles. — The lateral muscles of the abdomen 
do not conform so closely to a general plan of arrangement as do the dorsal 
and ventral muscles. Most of them are intrasegmental in position, 
and tergosternal in their attachments (Fig. 142 A, le) ; but some of them 
may lie on the intersegmental folds (li), and frequently some of them 
are intersegmental in the sense that they cross obliquely from one seg- 



Fig. 143. — Diagrams of abdominal musculature. A, cross section, showing principal 
muscles differentiated into distinct groups of fibers, Tvith lateral muscles comprising 
tergosternal (/-s), tergopleural {t-p), and sternopleural {s-p) muscles. B, illustrating 
lateral muscles differentiated into compressors (cpr) and dilators {dir)* C, longitudinal 
section through consecutive terga, showing usual position of external dorsals (c?e) and inter- 
nal dorsals (rfi), which are both retractors. D, same, with outer dorsals {de) reversed in 
position to function as protractors, (For other lettering see page 260.) 

ment to the next. Furthermore, the lateral muscles are not always 
strictly tergosternal in their attachments, for some of them may be 
attached at one end on small sclerites located in the pleural areas of the 
lateral integument between the tergal and sternal plates. Such muscles, 
therefore, may be termed tergopleural, or pleurosternal. A division of 
the lateral muscles into internal laterals and external laterals (Fig. 142 C, 
h, le) is not always apparent, often because of the absence of an internal 
group, but it is of common occurrence. 

In some insects there is a well-defined internal set of lateral muscles 
lying mesad of the lateral longitudinal tracheal trunk, having the upper 
attachments on the dorsum above the paradorsal muscle (Fig. 142 C, li), 



THE ABDOMEN 


263 


when the latter muscle is present. The internal laterals may be dis- 
tributed along the length of each segment, but in some cases they are 
limited to the extreme anterior parts of the segments, and in certain 
holometabolous larvae they lie on the intersegmental folds (A, li). 
The external laterals (C, le) arise dorsally below the paradorsal muscle 
(p), when tills muscle is present. Frequently some of them cross each 
other obhquely, and in their attachments they are often diversified into 
tergosternal, tergopleural, and pleurosternal groups. 

Functionally, most of the lateral muscles are compressors of the 
abdomen (Fig. 143 B, cpr), since their contraction approximates the 
tergal and sternal plates. With some insects, however, in which 
the lateral parts of the abdominal terga overlap the edges of the sterna, 
certain of the lateral muscles are so situated as to be antagonistic to the 
others. These muscles, therefore, become dilators of the abdomen (B, 
dir). Their dilator action results from the fact that their tergal attach- 
ments are on the lower edges of the terga ventral to their sternal attach- 
ments. By contraction, therefore, they separate the tergal and sternal 
plates. The effectiveness of these muscles is usually increased by the 
elevation of their sternal ends on lateral apodemal arms of the sternal 
margins (Fig. 144 A, 177). The contraction of the lateral abdominal 
muscles most frequently produces a movement of the sternal plates; but if 
the sternal arcs are larger and more rigid than the tergal plates, it is the 
latter that respond to the action of the lateral muscles. IWien a dilator 
mechanism is absent, the expansion of the abdomen following contraction 
results from the general elasticity of the abdominal integument. Soft- 
skinned larval insects usually contract only a small part of the body at one 
time, and this part is then expanded by pressure resulting from contrac- 
tion in some other part. 

The Transverse Abdominal Muscles . — The transverse muscles of the 
abdomen are best known as the muscles of the dorsal and ventral dia- 
phragms (Fig. 142 C, td, tv). The fibers of the dorsal diaphragm arise 
typically in groups on the anterior edges of the lateral parts of the 
abdominal terga and spread mesally to their attachments along the ven- 
tral wall of the heart. In a few insects they are evenly distributed along 
the entire length of each tergum or collected into anterior and posterior 
groups. The ventral transverse muscles in some insects, as in Acrididae 
and Hymenoptera, form a continuous sheet of weblike tissue through- 
out most of the length of the visceral region of the abdomen, which 
constitutes a ventral diaphragm stretched between the edges of the 
sterna over the ventral nerve cord; in others, however, as in Tettigoniidae 
and most Gryllidae, the ventral fibers are aggregated to form widely 
separated compact muscles crossing the anterior parts of the abdominal 
sterna. Not only are the ventral transverse muscles more variable in 



264 


PRINCIPLES OF INSECT MORPHOLOGY 


their arrangement than are their dorsal counterparts, but they are of less 
constant occurrence and are generally absent in holometabolous larvae. 

The Spiracular Muscles of the Abdomen . — ^The regulator mechanism 
at the entrance to the abdominal tracheae usually includes one or two 
muscles associated with each spiracle. The muscle most generally 
present is an occlusor. This is a short muscle usually attached at both 
ends on apodemal processes of the spiracular atrium, where its contraction 
compresses the inner end of the atrium and so closes the entrance to the 
trachea; in the Acrididae the occlusor muscle arises dorsally on the tergum 
close behind the spiracle. An antagonistic muscle, or dilator of the 
spiracle, is absent in many insects; when present it arises ventral to the 
spiracle, on either the tergum or the sternum, and is inserted on one of 
the processes of the atrium in line with the occlusor. The regulator 
mechanism of the spiracles will be more fully described in Chap. XV on 
the respiratory system. 

The Abdominal Musculature of a Grasshopper. — The abdominal 
musculature will be best understood by studying the muscles of some 
fairly generalized insect, and any of the larger grasshoppers will serve as a 
good subject for laboratory work. 

The abdominal muscles are well developed in the Acrididae, since 
the grasshoppers make dorsoventral expansions and contractions of the 
abdomen during breathing and execute strong movements in this part of 
the body during the acts of copulation and oviposition. The great 
ex-tension of the female abdomen during oviposition, however, is appar- 
ently caused by the action of the muscles connected with the ovipositor; as 
the latter organ automatically digs into the earth, it stretches the visceral 
region of the abdomen far beyond the capacity of the protractor muscles. 
The muscular actmties of the abdomen are all accomplished by the 
abdominal muscles, there being no muscles in the grasshopper extending 
from the thorax into the abdomen. The abdominal musculature shows 
little variation in the several segments of the visceral region, except 
in the first and second segments (Fig. 144 B). The muscle pattern in the 
third segment (A) may be taken as typical of the general segmental plan 
of the abdominal musculature; but in the genital segments the muscula- 
ture is highly modified, and in the terminal segments it is reduced. For a 
general review of the abdominal musculature in orthopteroid insects the 
student is referred to the work of Ford (1923). The following specific 
descriptions are based on Dissosieira Carolina, the abdominal muscles of 
which the writer has fully described elsewhere (1935). 

The Dorsal Muscles . — The dorsal muscles of the grasshopper occupy 
the lateral areas of the abdominal terga, but they do not entirely cover 
the tergal surfaces. The internal dorsals form several broad bands of 
fibers in each side of the body (Fig. 144 A, 167, 168, 169) and are in general 



THE ABDOMEN 


265 


longitudinal though they have a tendency to obliquity, which is accen- 
tuated in the more posterior segments (B). The most lateral group of 
dorsal fibers on each side (A, 169) is a paradorsal muscle, since it is 
separated from the others by the upper ends of the internal lateral muscles 
{175, 176). The others, again, are divided by the attachments of the 
muscles of the dorsal diaphragm on the body wall (A, td) into a median 
intrapericardial group of three or four flat bands of fibers {167a, b, c, d,), 
and into a broad, lateral extrapericardial muscle {168). The intraperi- 
cardial fibers are attached anteriorly on a secondary tergal ridge {tr); 



Fig. 144. — Abdominal musculature of a grasshopper, Dissosteira Carolina. A, muscles 
of right half of third segment. B, muscles of right half of segments I to V. 

posteriorly all the dorsal muscles are inserted on the anterior margin of 
the following tergum {IV T). The internal dorsal muscles are thus 
retractors of the terga. The wide separation of the ends of corresponding 
groups of the longitudinal fibers in consecutive segments of the grass- 
hopper presents an atypical condition. 

The external dorsals comprise two muscles in each segment, one 
median (Fig. 144 A, 170), the other lateral {171), which assume oblique 
or transverse positions. In the third segment each of the external 
dorsals arises on the posterior part of the tergum, the median one {170) 
extending dorsally to its insertion on the anterior edge of the following 
tergum, the lateral one ventrally {171). In the more posterior segments 
the corresponding muscles are longer and cross each other on the side of 
the tergum. The external dorsals of the grasshopper are thus torsion 
muscles serving to give a partial transverse rotation of the abdominal 
segments on each other. 

The Ventral Muscles.— The ventral muscles form a uniform series in 
the first seven segments of the female and in the first eight segments of 
the male. The internal ventrals are distinctly divided in each segment 
into a broad median band of longitudinal fibers (Fig. 144 A, 172) reaching 



266 


PRINCIPLES OF INSECT MORPHOLOGY 


from a submarginal sternal ridge (sr) to the anterior edge of the following 
sternum, and into a smaller bundle of lateral fibers {173) extending from 
the anterior lateral area of the sternum to the anterior end of the anterior 
apophysis {aAp) of the following sternum. Both sets of internal ventrals 
are sternal retractors. 

The ex-ternal ventral muscles consist of a single bundle of fibers on 
each side of each segment (Fig. 144 A, 174). Each muscle arises on the 
posterior lateral area of the sternum of its segment and extends anteriorly 
to its insertion on the overlapping under surface of the anterior apophysis 
{aAp) of the succeeding sternum. The external ventrals are thus pro- 
tractor muscles inasmuch as their contraction serves to separate the 
sternal plates. Since there are no tergal protractors, the sternal pro- 
tractors e\ddently may give an upward flexure to the extended abdomen 
or serve to counteract a deflexed condition produced by the internal 
ventrals in opposition to the internal dorsals. 

The Lateral Muscles . — ^The arrangement of the lateral muscles forms 
the same pattern in segments III to VII (Fig. 144 B), in which all the 
lateral muscles are tergosternal in their attachments. There are two 
internal laterals in each side of each of these segments (A, 175, 176), both 
arising on the tergum beneath the ventral edge of the lateral dorsal 
muscle {168)] the first {175) is inserted on the base of the lateral sternal 
apodeme {lAp), the second {176) on the lateral margin of the sternum. 
External to the second internal lateral are two oblique outer laterals 
{178, 179) having their origins on the tergum ventral to the paradorsal 
muscle {169)] the two cross each other, going respectively posteriorly and 
anteriorly to their insertions on the lateral edge of the sternum. The 
internal laterals and the two oblique external laterals are all compressors 
of the abdomen and are therefore expiratory muscles in respiration. A 
third external lateral arises from the lower anterior angle of the tergum 
(A, 1 77) and extends dorsally to its insertion on the upper outer surface 
of the lateral sternal apodeme {lAp). This reversed lateral is antagonistic 
in its action to the other laterals ; it is therefore a dilator of the abdomen 
and an inspiratory muscle in respiration. 

In the first abdominal segment the lateral musculature is reduced 
to a single slender muscle (Fig. 144 B, 146), which is apparently a tensor 
of the tjTOpanum of the “auditory” organ. In the second segment the 
lateral muscles do not entirely conform with those of the segments follow- 
ing, and in addition to the tergosternal muscles there are a pair of tergo- 
pleural muscles and a single short sternopleural muscle; these lie external 
to the tergosternal laterals and are therefore not seen in the figure. 

The Complex Types of Abdominal Musculature. — The musculature of 
apterj'gote hexapods is not well known in all the several groups, but 
it has been carefully studied in Protura and Japygidae. The abdominal 



THE ABDOMEN 


267 


musculature of Protura, as described by Berlese (1910), is somewhat 
more complex than that of adult pterygotes. In the Japygidae the mus- 
culature throughout the body presents a highly complicated pattern; 
in each of the first eight abdominal segments of Heterojapyx, for example, 
there are at least 40 pairs of muscles having a most intricate arrangement, 
which, except that the fibers are comprised in dorsal, ventral, and lateral 
sets, shows little to suggest that the muscle pattern of adult pterygote 
insects has been derived from it. The multiplicity of muscles in Hetero- 
japyx would appear to be a specialized condition. Among holometa- 
bolous larvae the body musculature is elaborate in the maggots of higher 
Diptera, but it reaches its greatest degree of complexity in the caterpillars 



Fig. 145. — Ventral muscles and muscles of right half of the mesothorax and metathorax 
of a caterpillar, Malacosoma americana, inner view. 

(Fig. 145). In more generalized forms, however, the abdominal muscular 
ture of the larva is not essentially different from that of the adult. 

3. THE ABDOMINAL APPENDAGES 

The usual abdominal appendages of adult insects are the gonopods of 
the genital segments and the uropods, or cerci, of the eleventh segment. 
In some of the Apterygota, however, appendages occur also on the pre- 
genital segments, and the larvae of Pterygota present numerous varieties 
of appendicular structures on the abdomen, many of which appear to be 
rudiments of true segmental limbs. The cerci have already been 
described in connection with the eleventh segment (page 255), the gono- 
pods will be discussed in Chap. XIX on the external genital organs; 
the present section, therefore, is limited to a brief review of the pregenital 
appendages of the Apterygota and the appendicular organs of pterygote 
larvae. 


268 


PRINCIPLES OF INSECT MORPHOLOGY 


Abdominal Appendages of Protura— A pair of short cylindrical 
appendages is present on each of the first three abdominal segments of 
adult Protura, arising from the membranous parts of these segments 
between the posterior angles of the tergal and sternal plates. The 
appendages are best developed in Eosentomidae, where the three pairs 
are alike in size and structure; each organ consists of two segments (Fig. 
146) and a small terminal vesicle (a), which is eversible and retractile. 

In Acerentomidae the appendages of 
the first pair are like those of the 
Eosentomidae, but the second and 
third pairs are very small, simple 
tuberculiform protuberances, unseg- 
mented and lacking the vesicle. 
Each appendage of the larger type in 
the two families, as described by 
Berlese (1910), is movable by two 
tergal muscles (B, I, J) inserted on 
the basal segment, one anteriorly, the 
other posteriorly. The second seg- 
ment is provided likewise with two 
muscles, one arising anteriorly, the 
other posteriorly in the proximal seg- 
ment, the two crossing each other 
medially to be inserted on opposite sides of the base of the distal segment. 
The terminal vesicle is retracted by a single large muscle (rv), which takes 
its origin mesally in the base of the first segment and is inserted on a cen- 
tral depression of the ventral face of the vesicle. The extrusion of the 
vesicle is evidently brought about by blood pressure from within the 
body. 

Abdominal Appendages of Collembola. — The Collembola have three 
characteristic appendicular organs on the abdomen, which, though 
unpaired at least basally in the adult stage and located medially on the 
ventral side of the body, are said to be formed in the embryo from paired 
rudiments. Each retains in its adult structure evidence of its double 
origin. The first appendage is carried by the first abdominal segment and 
is known as the ventral tube, or collophore (Fig. 147 A, Coll) ; the second is 
the clasp, or tenaculum (C), of the third segment; the third is the spring, 
called the furcula (A, Fur), apparently arising from the fifth abdominal 
segment, though its muscles take their origin in the fourth. 

The Collophore. — The ventral tube, or collophore, is a large, thick 
cyhndrical pouch of the body wall projecting ventrally and somewhat 
anteriorly from the sternal region of the first abdominal segment (Fig. 
147 A, Coll). In most species the tube ends in a bilobed terminal vesicle 



Fig. 146. — Abdominal appendages of 
Protura. A, abdominal leg of Eosen- 
tomon germanxcum. (From Prell, 1913.) 
B, musculature of first abdominal leg of 
Acerenlomon doderoi. (From Berlese, 
1910.) 



THE ABDOMEN 


269 


(B, r), ^vllich is ordinarily retracted but is capable of being protruded by 
blood pressure. A pair of large lateral retractor muscles (rv), arising 
within the bods'-, traverse the collophore to be inserted on the lobes of the 
terminal vesicle. The structure of the collophore thus suggests that the 
organ is formed by the fusion of a pair of abdominal appendages resem- 
bling those of the Protura (Fig. 146), though in the latter the retractor 
muscle of the vesicle (rv) is said to arise in the base of the appendage, 
while the appendage itself is movable by two muscles (I, J) arising in the 
body and inserted on its base. In some of the Collembola, as in Smin- 
thuridae, each lobe of the vesicle is produced into a long eversible tube. 

The anterior surface of the collophore presents a median vertical 



Fig. 147. — Abdominal appendages of a collcmbolan, Tomoccrus •vulgaris. A, entire 
insect with furcula in flexed position. B, collophore. C, tenaculum. D, furcula. 

groove continuous vcntrally with the depression between the lobes of the 
terminal vesicle and communicating dorsally with a median channel of the 
ventral wall of the thorax. Anteriorly the thoracic channel is continued 
upon the outer face of the rudimentary labium as far as the distal cleft 
of the latter, where there open into it the ducts of two pairs of head glands. 
It has been suggested by Willem and Sabbe (1897) that the secretion of 
the head glands is conveyed to the collophore tlirough the ventral thoracic 
channel and, when collected between the lobes of the terminal vesicle, 
enables the tube to function as an adhesive organ. Hence the name 
collophore (“glue bearer”). An elaborate description of the histology 
of the collophore, the head glands, and the connecting channel is given by 
Hoffmann (1905), who concurs with Willem and Sabbe as to the function 
of the tube. An adhesive function of the organ, however, has apparently 
not been demonstrated, and it is quite possible that the channel between 
the vesicle and the labium might convey liquid from the former to the 
mouth; and yet, though the Collembola inhabit moist places, most of 
them do not ordinarily come in contact with water. 



270 


PRINCIPLES OF INSECT MORPHOLOGY 


The Tenaculum. — The tenaculum, or clasp, is a minute organ situated 
medially on the concave ventral surface of the third abdominal segment. 
It consists of a conical base and of two laterally divergent distal prongs 
toothed on their outer margins (Fig. 147 C). Each prong is provided 
with an adductor muscle (ad). The tenaculum serves to hold the furcula 
in place when the latter is flexed against the ventral side of the body, the 
prongs projecting between the bases of the furcular arms. 

The Furcula. — ^The furcula is the leaping organ of the Collembola and 
is the feature from, which the insects get their common name of “spring- 
tails.” The furcula consists of a large median base, the manubrium 
(Fig. 147 D, mn), and of two slender arms, each of which is subdivided 
into a long proximal segment, the dens (d), and a short terminal segment, 
or mucro (m). On the base of the manubrium are inserted a pair of 
flexor and a pair of extensor muscles arising in the fourth abdominal 
segment. In Tomocerus vulgaris each of the arms is provided with an 
abductor muscle (ab) and an adductor muscle (ad) having their origins 
in the manubrium. When the furcula is flexed in the position of repose 
(A), its proximal half is concealed in a ventral concavity of the abdomen, 
and the arms are closed upon the tenaculum, which fits into an oval space 
between the bases of the dentes (D, a) having thin, hard edges that are 
held by the teeth on the outer margins of the tenacular prongs (C). 
The spring evidently is released by the contraction of the adductor mus- 
cles of the prongs (C, ad). At the same time the furcular arms are spread 
and the entire organ is forcibly extended, throwing the insect upward and 
forward. 

The furcula varies much in length in different species of Collembola, 
and it is absent in the genera Neanura and Anurida. In some species of 
Sminthurus that live on the surface of water the spring has a fanlike 
structure, the divergent arms being fringed with long, stiff hairs. 

Abdominal Appendages of Thysanura. — The abdominal appendages 
of Thysanura are of particular interest because they have been taken 
as a starting point for the study of the structure of the genital appendages 
of adult pterygote insects, and they appear also to retain the basic struc- 
ture of the abdominal appendages of pterygote larvae. 

The thysanuran abdominal appendages, not including the cerci, best 
preserve their individuality in Macliilidae, where they are present on each 
of the first nine segments of the abdomen except the first. Each of the 
pregenital appendages in this family consists of a large lateroventral 
basal plate, or rather of a flat basal lobe having a wide plate in its ventral 
wall (Fig. 148 B, Cxpd), and of a distal tapering process, termed the 
stylus (Shj), which is freely movable on the basis. The basal plates of 
each segment are intercalated proximally between the deflected lateral 
edges of the tergum (Fig. 138 A, IT) and the small triangular median 



THE ABDOMEN 


271 


sternum {Stn)] they are united with the sternum, and ankylosed with 
each other medially behind the sternum. Each plate is provided with 
muscles arising on the tergum and has all the aspects of being the enlarged 
basis of an otherwise rudimentary limb. 

The abdominal styli of Thysanura are equipped mth muscles arising 
proximally in the basal plates (Fig. 148 B, smcls). They would appear, 
therefore, to be the rudimentary telopodites of the abdominal append- 
ages. In the Machilidae, however, similar styliform processes occur on 
the coxae of the second and third thoracic legs (A), which, though they 
lack muscles, suggest by their form that they are serially homologous 
with the abdominal styli. It is possible, therefore, that both the thoracic 
and abdominal styli are coxal epipodites; they are not “exopodites,” as 
they are often supposed to be, since the true exopodite of Crustacea (Fig. 




Fig. 148. — Appendages of Aptcrygota. A, motathoracic leg of Nesomachilis wth 
coxal stylus. B, abdominal appendage of NesomaeMlis bearing a stylus (Slv) and retractile 
vesicle (Fs). C, abdominal appendage of Helerojapyx with base (Cxpd) united with 
sternum. 


44 C) arises from the first trochanter (basipodite). The abdominal styli 
of Thysanura support the abdomen of the insect in life and play an active 
part in locomotion. Whatever may be their morphological nature, the 
styU appear to be represented in various forms on the abdominal seg- 
ments of pterygote larvae and on the male genital segment of many adult 
insects. 

Styli are present on the posterior segments of some Thysanura in 
which the limb bases are fused with the sterna, and they occur likewise 
on lateral lobes of the definitive pleurosternal plates of Diplura (Fig. 148 
C). There can be little question that the styli present on the ninth 
sternum of certain male Pterygota, such as Ephemeridae, Termitidae, 
Blattidae, Grylloblattidae, and Tettigoniidae (Fig. 138 C), are homo- 
logues of the thysanuran styli, and it seems equally certain that the 
movable genital claspers of male holometabolous insects are organs 
equivalent to the more typical styli of these less specialized pterygote 
insects 




272 


PRINCIPLES OF INSECT MORPHOLOGY 


A second distal structure of the pregenital appendages, present in 
most of the Thysanura, has the form of a small eversible and retractile 
vesicle (Fig. 148 B, 7s) located mesad of the base of the stylus, and 
provided with strong retractor muscles [rvs) arising proximally on the 
basal plate. In some species there is a pair of vesicles on each appendage. 
The function of these organs is not known. Their structure suggests 
that they represent the terminal vesicles of the pro- 
turan abdominal appendages (Fig. 146, v)', but the 
presence of two sacs on each appendage in some 
species of Thysanura precludes the idea that they 
may be rudiments of the distal parts of the abdominal 
limbs. The gill-bearing tubercles on the bases of the 
abdominal limbs of the larva of the neuropteron 
Corydalus and the terminal lobes of the abdominal 
legs of caterpillars have a structure very similar to 
that of the retractile vesicles of the Thysanura. 

The gonopods of Machilidae and Lepismatidae 
differ from the pre genital appendages in that each 
may be provided with a gonapophysis and always 
lacks a retractile vesicle (Fig. 313 A, B, C). The 
gonapophysis (B, C, Gon) is a slender process arising 
from the mesal proximal angle of the coxopodite and 
is provided with short muscles arising in the latter 
(gmcl). Its proximal position on the coxopodite 
shows that it has no relation to the vesicles of the 
pregenital appendages and also does not favor the idea 
that the gonapophyses are the telopodites of the geni- 
tal appendages. The four gonapophyses form the ovi- 
positor of the female. The first pair is usually absent 
in the male, and in some species neither pair is present. 

Abdominal Appendages of Pterygote Larvae. — The larvae of pterygote 
insects are remarkable for the variety of appendicular organs they have 
on the abdomen. Morphologists have not given much attention to these 
structures because it has been supposed that they are special develop- 
ments ser^^ng the needs of the larvae; but there is no question as to the 
origin of some of them from the limblike rudiments of the embryo, and 
nearly all of them suggest by their structure and musculature that they 
are parts at least of true segmental appendages. 

The most leglike in form of the larval abdominal appendages occur in 
the neuropterous genus Sialis and on certain aquatic coleopterous larvae, 
especially in the families Dytiscidae and Gyrinidae. The larva of 
Sjahs (Fig. 149) has on each of the first seven segments of the abdomen 
a pair of long, tapering, si.x-segmented appendages projecting laterally 



Fig. 149. — Larva 
of Sialis, showing leg- 
like appendages of the 
abdomen. 



THE ABDOMEN 


273 


from the sides of the body. Each appendage (Fig. 151 D) is supported 
on a lateral lobe (Cxpd) of the body wall of its segment, and within this 
lobe there arise muscles inserted anteriorly and posteriorly on the base 
of the movable shaft of the appendage. More than this, there are mus- 
cles within the proximal part of the appendage itself. The development 
of these organs in the embryo has not been studied, so far as the writer is 
aware. The abdominal appendages of the coleopterous larvae men- 
tioned above are very similar to those of Sialis. The appendages in all 
cases are penetrated by tracheae and are supposed to function as gills, 
but this assumption needs experimental evidence. 

The well-known gills of ephemerid larvae are borne on lobes on the 
sides of the abdominal segments (Fig. 150 A, B, Cxpd) situated between 



Fig. 150. — Abdominal appendages of an ephemerid larva. A, diagrammatic cross 
section of an abdominal segment, showing pleural lobes {Cxpd) of the body bearing the gills 
{Brn). B, a gill and its basis {Cxpd) in the pleural wall of the body. C, the gill muscles. 


the terga and the sterna, and each gill is provided with muscles inserted on 
its base which arise in the ventral part of the supporting lobe (B, bmcls). 
There is little doubt, therefore, that the gills are appendicular parts 
of abdominal lim bs, of which the supporting lobes are the bases. The 
gill stalk or gill plate, by its position on the basis and its basal muscula- 
ture, suggests that it is a homologue of the stylus of the thysanuran 
abdominal appendages. The gill basis is very evidently the equivalent 
of the stylus-bearing plates of Machilidae, though, since it is immovable, 
there are no body muscles inserted upon it. 

Returning again to the neuropterous family Sialidae, we find in the 
genera ChauUodes and Corydalus long tapering appendages on the sides of 
the first eight abdominal segments, and a terminal pair (pygopods) on 





274 


PRINCIPLES OF INSECT MORPHOLOGY 


the tenth segment (Fig. 151 A, B, C). Each appendage is a hollow 
process of the integument and is supported on a lateral lobe of the body- 
wall (C, E, Cxpd). The appendage bases fall in line with the thoracic 
subcoxae, and within them arise muscles inserted on the bases of the 
movable parts of the appendages (F, smcls). It would appear, therefore, 
that we have here also a reduced and modified limb consisting of the 
coxopodite (Cxpd) and a distal part (Sty), the latter representing the 
ephemerid gill, or the thysanuran stylus. In Corydalus the basis of each 
of the first seven pairs of appendages supports ventrally a large tubercle 
bearing a thick tuft of gill filaments (C, E, Fs). A long muscle (E, F, rvs) 
arising on the dorsum of the body segment is inserted by three branches 



Fig. 151. — Abdominal appendages of larvae of Neuroptera. A, end of abdomen o' 
Chauliodcs, dorsal view. B, same, ventral -view. C, Corydalus cornutus, segments VII-X 
and appendages. D, appendage of Sialis. E, an abdominal segment of Corydalus, pos- 
terior -view. F, right half of same in section showing muscles of appendage. G, tenth 
segment appendage (pygopod) of Corydalus, right, anteromesal view. H, claws of terminal 
appendage of Corydalus and their retractor muscle. 


in the distal end of the tubercle and cAudently serves to retract the latter. 
The gill-bearing tubercles of sialid larvae thus recall, in their structure and 
musculature, the retractile vesicles of the abdominal limbs of Thysanura, 
though in the sialid the tubercle muscles arise on the dorsum of the body 
and not in the appendage bases. 

The appendages of the tenth larval segment, or pygopods, in both 
Cliaxdiodcs and Corydalus, differ from those of the preceding segments in 
that the basis of each projects from the body as a short, free cylindrical 
lobe (Fig. 151 A, B, C, Ppd, G) bearing the stylus laterally (C, G, Sty), 
and the tubercle is pro^^ded with twm large eurved claws (d) instead of 
gills. The claws are set on the flat distal end of the tubercle by long. 




THE ABDOMEN 


275 


parallel bases, and the retractor muscle (H, rvs) is inserted posteriorly 
at the proximal ends of the convex margins of the claws. 

The larvae of Trichoptera likewise have a pair of large claw-bearing 
pygopods on the tenth abdominal segment, though they have no append- 
ages on the other segments of the abdomen. In some forms these 
terminal appendages are short, each consisting of a decurved distal claw 
arising from two basal plates implanted on the side of the tenth segment 
(Fig. 152 B, Ppd, C). In others the appendages are long, freely movable, 
cylindrical organs, projecting posteriorly, each bearing a large decurved 
claw on its distal end (E, P). Neither the structure nor the musculature 
(D, E) of the pygopods of trichopterous larvae in any way resembles 



Fig. 152. — Abdomen and appendages of larvae of Trichoptera. A, Platyphylax 
designatua, metathorax and base of abdomen. B, end of abdomen. C, same, left pygopod 
of tenth segment. D, same, right pygopod and muscles. E, Hydropayche, right pygopod 
and muscles. F, same, end of abdomen, with pygopods, and intestinal filaments protruded 
from the anus. 


that of the terminal appendages of the sialid larvae, nor do they have any 
similarity to the terminal legs of lepidopterous larvae; this fact is some- 
what surprising considering that in many respects the Trichoptera and 
Lepidoptera appear to be related orders. Clawlike appendages are 
present on the tenth segment of certain coleopterous larvae, as in the 
family Helmidae, but the morphological status of such structures is 
doubtful. 

The abdominal legs of lepidopterous larvae are said by students of 
embryology to be developed from limb rudiments in the embryo that 
correspond to the rudiments of the thoracic legs. They appear, there- 
fore, to be true segmental appendages. Most caterpillars have five pairs 
of these abdominal legs (Fig. 153 A), four pairs being on segments HI to 



276 


PRINCIPLES OF INSECT MORPHOLOGY 


VI, inclusive, the fifth on segment X; but the number is sometimes 
reduced, as in the loopers, and in some forms all the appendages are 
suppressed. 

A typical abdominal leg of a caterpillar consists of three parts (Fig. 
153 F). Proximally there is a ring of flexible integument (mb); beyond 
this is a longer cylindrical section (Cx) forming the greater part of the 
appendage, and usually having a sclerotic plate in its outer wall, often 
marked by a distinctive group of setae (A); distally the leg ends in a 
retractile lobe (F, Fs), called the plania, which bears the claws, or crochets 
id). 

Functionally the planta is the most important part of the abdominal 
leg of the caterpillar, and structurally it is the most variable. In its 
more generalized condition the planta is a short circular pad (Fig. 153 
B, Fs) with a central depression (e) on which is inserted a group of retrac- 
tor muscle fibers (rvs). In such cases the crochets (d) may be arranged 
in a complete circle around the periphery of the distal plantar surface, 
with their recurved points turned outward and upward. With most 
caterpillars, however, the claws are limited to a semicircle or a small arc 
usually on the inner margin of the planta (C, D), and in such cases the 
planta itself (Fs) generally becomes asymmetrical by a reduction or 
obliteration of its outer half. The planta then assumes the form of a 
lobe projecting to the mesal side of the limb axis, the latter being marked 
by the insertion point of the retractor muscle (e), and the crochets curve 
mesally and upward when the planta is protracted in the usual position 
(H). 

Immediately above each abdominal leg of the caterpillar there is 
usually a prominent lobe or swelling of the body wall (Fig. 153 A, F, 
Sex), limited above by a groove marking the dorso-pleural line (a-a) of the 
abdomen. Corresponding lobes are present on the legless segments of the 
abdomen, and also on the thorax above the bases of the legs. The series 
of suprapedal lobes, therefore, appears to represent the lateral parts of 
the subcoxae on both the thorax and the abdomen (A, Sex). 

The musculature of an abdominal leg of a caterpillar consists of two 
sets of fibers (Fig. 153 I), those of one set being inserted on the base of 
the principal part of the leg (Cx), those of the other on the distal surface of 
the planta (Fs). The plantar muscles, in the species figured, consist of 
four fibers, three of which (5, 6) arise in the upper part of the subcoxal lobe, 
while the fourth (4) arises on the lateral wall of the body segment. The 
insertion of the other muscles {1, 2, 3) on the base of the principal segment 
of the log (Cx) suggests that the latter is the coxa; the musculature of the 
planta leaves little doubt that the planta is a structure equivalent to 
the gill-bearing tubercles of the neuropterous larvae above described 
(Fig. 151 F), and that it is therefore analogous at least to the retractils 



THE ABDOMEN 


277 


vesicles of Thysanura. In the caterpillars most of the plantar muscles 
arise in the limb base, but there is always a long fiber from the body 
wall; the plantar musculature is thus intermediate between that of the 
vesicles of Thysanura and the gill tubercles of Neuroptera. Representa- 
tives of styli are not present in any lepidopterous larvae. 

The appendages of the tenth abdominal segment of the caterpillar, 
known as the anal legs, or postpedes, resemble the appendages of the pre- 



Fig. 153. — Abdominal appendages of larvae of Lepidoptera. A, Carpocapsa pomo- 
nella. B, same, left abdominal leg, ventral view. C, Hyphaniria cunea, left leg, ventral 
view. D, Xylina, right leg, ventral view, E, diagrammatic section of caterpillar grasping 
a twig. F, Malacosoma americana, left leg, posterior view. G, diagram of position of 
planta on a rough surface. H, same on a smooth surface. I, Malacoaoma americana, 
right leg and muscles, posterior view. 


ceding segments in structure; their musculature differs from that of the 
others in that the basal muscles are largely eliminated, while the muscles 
of the planta are much larger and include both dorsal and ventral groups 
of fibers. 

When a caterpillar with lobate plantae clings to a small twig or plant 
stem, the abdominal feet are turned mesally and clasp the support with 
the incurved claws (Fig. 153 E). The closure of each pair of legs on 
the support must be caused by the contraction of the median muscles (1) 
inserted on their bases, for the plantar muscles {rvs) evidently serve to 
release the grasp of the claws. If the caterpillar walks on a flat but 




278 


PRINCIPLES OF INSECT MORPHOLOGY 


rough surface, the plantar lobes are turned outward by their muscles (G) 
and their inner surfaces are applied to the support with the claws directed 
downward. If, however, the caterpillar finds itself on a smooth, hard 
surface, such as that of glass, the soles of the plantae are pressed flat 
against it (H), with the claws turned upward, and apparently a tension 
of the plantar muscles converts the soft end walls of the plantae into 
vacuum cups by which the caterpillar maintains its foothold. 

The caterpillars do not move either their abdominal or their thoracic 
appendages in the way that adult insects move their thoracic legs. In 



Fig. 154. — Terminal abdominal appendages and other appendicular processes of larvae 
of Hymenoptera and Coleoptera. A, Pleronidea ribesit, end of abdomen with pygopods. 
B, Cimbex americana. C, Ccphaleia. D, Codes helopiodes, end of abdomen with uro- 
gomphi. (From Kemner, 1918.) E, Thanatophilus. (From Kemner, 1918.) F, Dytiscua 
drcumcinctus, end of abdomen with appendicular processes. 

regular forward progression the last pair of abdominal legs are first 
released from the support and brought forward by a contraction and slight 
humping of the posterior part of the body. Then in turn the other 
abdominal legs are lifted and advanced in the same manner, as the wave 
of body contraction runs forward through the segments. Finally, the 
movement affects the thoracic segments and their appendages. Thus 
the crawling caterpillar moves forward with each successive wave of 
contraction that runs through its body. If the caterpillar is a “looper,” 
the posterior group of appendage-bearing segments is brought forward 
together and the body is stretched out for a new grasp by the thoracic 
legs. 

The larvae of the sawflies (Tenthredinidae and related families) 
resemble caterpillars in the possession of appendages on the abdomen, 
which are similar to those of the caterpillars but not so highly organized. 
The appendages of the tenth segment, however, differ in different forms. 
Those of species li\dng in the open (Fig. 154 A, B) are much the same as 
the anal legs of lepidopterous lanme and are adapted to grasping the 



THE ABDOMEN 


279 


edges of leaves; species that bore into the stems of plants or that live in 
the protection of web nests or curled leaves, however, such as the Cephi- 
dae and Pampliiliidae, have slender, jointed appendages on the tenth 
segment (C, Ppd). Abdominal appendages similar in appearance to 
those of lepidopterous and tenthredinid larvae occur also on some larvae 
of Coleoptera. 

Finally, we may mention, in connection with the study of abdominal 
appendages, certain fixed or mobile processes found on the terminal 
segments of certain larvae. Such structures are of frequent occurrence on 
the dorsum of the ninth segment in the larvae of Coleoptera and have 
been variously called styli, cerci, pseudocerci, and corniculi, but the 
term iirogomphi (Boving and Craighead, 1932) is more specific and 
descriptive. The urogomphi vary much in size and shape from short 
spine-like points to long, thick processes or multiarticulate filaments, and 
they are sometimes distinctly jointed (Fig. 154 D, E, ug). In some 
species they are fixed outgrowths of the posterior end of the ninth tergum; 
in others they arise from the membrane behind the tergal plate and are 
then flexible at their bases. Evidently the urogomphi are simply cutic- 
ular outgrowths of the dorsum of the ninth segment having no relation to 
segmental appendages. The terminal appendages of the larva of Dytiscus 
(F, ug?), however, are of a more problematical nature; they appear to 
belong to the ninth segment and are provided with muscles arising on the 
tergum of the eighth segment (Speyer, 1922; Korschelt, 1924); but it is 
possible that these appendages also are urogomphi, and that the muscles 
that move them are the intersegmental muscles between the eighth and 
ninth segments. 

The larvae of some chalastogastrous Hymenoptera have a pair of small 
processes arising on the tergum of the tenth segment (Fig. 154 A, a) which 
have sometimes been regarded as rudimentary cerci, but which are 
evidently mere cuticular processes comparable with the urogomphi of 
coleopterous larvae. In certain forms there is only a single median 
process (C, b). The caudal horn of sphingid caterpillars is an analogous 
structure. 



CHAPTER XII 


THE ORGANS OF INGESTION 

The organs primarily concerned with the intake of food are the 
gnathal appendages and lobes of the head surrounding the oral aperture 
of the alimentary canal, known collectively as the mouth parts. But 
the mouth parts do not constitute the entire apparatus of ingestion, for 
when the food has been delivered into the mouth cavity it must yet be 
passed on to the section of the alimentary canal where digestion takes 
place. The anterior part of the stomodaeum, then, is always an impor- 
tant part of the ingestive system. In the sucking insects, whose food 
consists mainly of plant and animal juices, the pumping apparatus asso- 
ciated with the mouth is principally a highly specialized development of 
the preoral cibarium; but, in its anatomical continuity, the pump becomes 
\drtually a part of the alimentary tract and is usually called the 
"pharynx.” In order to understand the true morphology of the ingestive 
organs, therefore, it will be necessary to refer back, on the one hand, to 
the contents of Chap. VII for the basic structure of the mouth parts, 
and, on the other, to anticipate something from the subject matter of the 
following chapter on the alimentary canal. 

So diverse in form are the feeding organs in the various groups of 
insects specialized for obtaining particular kinds of food that the study 
of the mouth parts becomes a major subject in any course in entomology. 
A good system for classifying the leading types of mouth-part structure, 
therefore, will be of much assistance in understanding the various func- 
tional adaptations of the organs, and the student is referred to the tabula- 
tion of insect mouth parts on a functional basis given by Metcalf and 
Flint (1928), and more fully elaborated by Metcalf (1929). For mor- 
phological purposes, however, the mouth parts cannot be studied from a 
physiological standpoint, since very different types of structure are often 
adapted to similar modes of feeding, and great discrepancies in both 
structure and function have been independently evolved in adult and 
larval forms of the same orders. Hence, in the following discussion of the 
more specialized feeding organs of insects, the leading types of structure 
will be described as they occur in the ordinal groups. 

Since all the more generalized forms of modern insects have the mouth 
parts constructed for feeding on so-called solid substances, that is, on 
the vhole tissues of plants and animals rather than on their juices or 

280 



THE ORGANS OF INGESTION 


281 


liquid products, there is no question that the “orthopteroid,” or biting 
and chewing, type of mouth parts is the one from which the other types 
have been derived, as the more specialized forms clearly show in most 
cases by their own structure and development. The fundamental struc- 
ture of mouth parts of the biting type, having been fully described and 
illustrated in Chap. VII, need be given little attention in the present chap- 
ter, wherein vdll be discussed the more important modifications charac- 
teristic of the principal orders. 

1. THE PREORAL CAVITY 

Since the mouth parts of insects are closely assembled in their attach- 
ments on the head, they enclose between them a space which is often 
called the “mouth cavity,” and which functionally deserves this name; 
but inasmuch as this region lies entirely outside the oral aperture, it is 
more appropriately termed the preoral cavity (Fig. 155, Pro). In a strict 
sense, of course, it is not a ca-\dty at aU but merely an external space 
bounded anteriorly by the epipharyngeal wall of the labrum and clypeus, 
posteriorly by the labium, and laterally by the mandibles and maxillae. 
Within the preoral cavity lies the tongue-like hypophar 3 mix {Hpliy), 
which morphologically is a median lobe of the ventral wall of the gnathal 
region of the head. The true mouth of the insect is the anterior opening 
of the stomodaeum (Mth), which is located in the ventral wall of the head 
(in hypognathous insects) immediately beliind the clypeus and in front 
of the hypopharynx, where it is normally concealed between the bases 
of the mandibles. Correspondingly situated at the posterior end of the 
hypopharynx, between the latter and the base of the prementum, is the 
opening of the salivary duct (SIO). 

In the orthopteroid insects the preoral cavity is largely occupied by 
the hypopharynx (Figs. 60 A, 155, Hphy). Anteriorly, however, there 
is an open food meatus (Jm) between the hypopharynx and the epipharyn- 
geal wall of the labrum and clypeus, which leads up to the mouth {Mth ) ; 
and posteriorly there is a broad salivary meatus (sm) between the hypo- 
pharynx and the labium, at the inner end of which is the opening of the 
salivary duct (SIO). The food passage is closed laterally by the man- 
dibles, and its upper or inner part, lying proximal to the molar surfaces of 
the closed jaws, forms the preoral food chamber here named the cibarium 
{Cb). The salivary channel {sm) terminates in the salivary pocket, or 
salivarium {Slv), between the base of the hypopharynx and the base of 
the labial prementum. The cibarium and the salivarium are important 
elements in the feeding mechanism of nearly all insects; in the higher 
orders they are variously modified to form specialized organs for the 
ingestion of liquid food and for the ejection of saliva or other products of 
the labial glands. 



282 


PRINCIPLES OF INSECT MORPHOLOGY 


The Cibarium. — The cibarium of generalized insects (Fig. 155, Ch) is a 
part of the intergnathal preoral cavity (PrC) of the feeding apparatus. 
Morphologically it lies outside the true mouth {Mth), but functionally 
it is the “mouth cavity” of the insect and is so defined (Mundhohle) by 
Weber (1933). Its concave floor is formed by the adoral surface of the 
base of the hypopharynx, flanked by the suspensorial sclerites of the latter 
(Figs. 60, 155, HS) ; its roof, or anterior wall, is the epipharyngeal surface 
of the clypeus. The cibarium, in chewing insects, serves as a chamber in 
wliich the food material, pushed upward through the food meatus by the 
adduction of the jaws, is held at the base of the hypopharynx preparatory 
to being passed into the mouth. If a partially narcotized cockroach 
is offered a bit of moistened bread, a particle is seized between the man- 
dibles; after a few movements of the jaws the particle may be seen neatly 
stowed in the cibarial pocket at the base of the hypophar 3 mx, from which 
it presently disappears into the mouth. During feeding, a copious flow 
of saliva issues from the salivary channel on the labium and floods the 
tips of the mouth parts. 

On the inner sm-face of the epipharyngeal wall of the cibarium is 
inserted a pair of dilator muscles (Figs. 60 A, 155, dlcb) taking their origin 
on the clypeus. The cibarium is compressed by the contraction of the 
retractor muscles of the mouth angles (rao), which arise on the frons and 
are inserted on the oral branches of the suspensorial bars of the hypo- 
pharynx {HS). The contraction of these muscles accompanying the 
adduction of the mandibles pulls the hypopharynx forward and upward, 
and it is this movement of the hypophar 5 Tix apparently that forces the 
food from the cibarial chamber through the mouth into the buccal region 
of the stomodaeum, whence it is carried along by the peristalsis of the 
stomodaeal wall. The opposite movement of the hypopharynx is pro- 
duced by the contraction of the retractor muscles (Figs. 60 A, 84 B, 155, 
rhphj) arising on the tentorium and inserted on the lateral sclerites (w) of 
the hypopharyngeal base. 

In most of the sucking insects, particularly in Djdiscidae, Thysanop- 
tera, Hemiptera, and Diptera, the cibarium undergoes a remarkable 
transformation by which it is converted into the sucking pump of the 
feeding mechanism. By an extension and closure of the lateral lips of 
the true mouth aperture, the cibarium becomes a chamber partly or 
entirely enclosed vithin the head cavity; its distal opening into the food 
meatus (Fig. 155, /?n) is then the functional mouth. The dilator muscles of 
the cibarial pump are always the epipharyngeal muscles arising on the 
cljpeus (dlcb). On the other hand, in Lepidoptera and Hymenoptera the 
sucking pump includes the phar 3 mx, and its dilator muscles arise on 
the clypeal, frontal, and postfrontal regions of the cranium. The relation 
of the muscles to the parts of the ingestive tract will be shown in a 
following section. 



THE ORGANS OF INGESTION 


283 


The Salivarium. — In its simplest form the salivarium is merely the 
pocket ivJiere the posterior or ventral wall of the hypopharynx is reflected 
into the anterior or dorsal wall of the labial prementum (Fig. 155, Slv), 
into which opens the duct of the salivary glands {SID). On its dorsal 
wall is inserted a pair of dorsal salivary muscles (Is) taking their origins 
on the suspcnsorial sclerites (HS) of the hypophai’ynx or on the lateral 
walls of the hypopharjmx Avhen these sclerites are absent. On its ventral 
wall are inserted the salivary muscles of the labium, usually two pairs 
(2s, 3s) arising in the prementum. 



Fig. 155. — Sectional diagram of the head of an orthopteroid insect showing the general- 
ized stomodaeal and hypopharyngeal musculature. Br, brain; BuC, buccal cavity; 
Cb, Cibarium; Clp, clypous; cplr, compressor labri; Cr, crop; d/bc, dilator buccalij; dlcb, 
dilator cibarii; Idlphy, Sdlphy, first and second diiatores pharyngium; dlpphy, dilator post- 
pharyngialis; fm, food meatus; Fr, frons; FrGng, frontal ganglion; Hphy, hypopharynx; 
HS, hypopharyngeal suspensorium; Lm, labrum; Mih, mouth; Oe, oesophagus; Phy, phar- 
ynx; Pml, postmentum; PPhy, postcriorpharynx; PrC, prcoral (mouth) cavits’’; Prmt, pre- 
mentum; rao, retractor anguli oris; rhphy, retractor hypopharyngis; Is, Ss, 3s, muscles of 
salivarium; SID, salivary duct; SlO, salivary orifice; Slv, salivarium; sm, salivary meatus; 
SocGng, suboesophageal ganglion; Tnl, tentorium; w, basal sclerito of hypopharynx. 


The primitive form of the salivarium is well shown in some of the 
Orthoptera. In the Acrididae, for example, the organ is a simple salivary 
cup on the base of the prementum, into which fits a prominent knob on 
the base of the hypopharynx. In the mantis the pocket is produced 
into a long, flat, triangular pouch (Fig. 84 D, Slv) with the orifice of the 
salivary duct at its apex. The lateral margins of the pouch are strength- 
ened by two weakly sclerotic bars (w) connected distally with the basal 
angles of the hypopharynx. On these bars are inserted the two pairs of 
salivary muscles from the labium (2s, 3s). The dorsal, or hypopharyn- 
geal, wall of the pouch is somewhat concave (E), with a median fold on 
which is inserted a pair of wide dilator muscles (D, E, Is) that converge 
from the lateral walls of the hypopharynx. A similar structure is 



284 


PRINCIPLES OF INSECT MORPHOLOGY 


described by Walker (1931) in Grylloblaita, including the three pairs of 
muscles. In Gryllus the salivarium is narrowed to a short rigid tube 
(Fig. 84 C, Slv), with both the hypopharyngeal and labial salivary muscles 
inserted upon it. The basal bars of the hypopharynx (w) diverge from 
the mouth of the tube into the lateral walls of the hypopharynx, where, 
as in most Orthoptera (Fig. 60), they give attachment to the tentorial 
retractor muscles of the hypopharynx (rhphy). 

The salivary ejection apparatus becomes highly developed in the 
larvae of Lepidoptera as the silk press (Fig. 165, Pr). In the eaterpillar, 
however, the hypopharynx and the prementum are united in a median 
lobe supported by the postmentum and the maxillary stipites, on the 
extremity of which the duct of the silk glands opens through a hollow 
spine, the spinneret {Sr). It is evident, therefore, that the silk press is 
the salivarium enclosed by the complete union of the h 3 rpopharynx with 
the prementum. Both dorsal and ventral salivary muscles are present 
in the caterpillar (Fig. 165 E) as in Orthoptera. In the higher Hymen- 
optera the salivary duct terminates in a cylindrical pouch opening above 
the distal part of the prementum (Fig. 163 C, Syr) just before the base of 
the rudimentary h 3 ^opharynx (Hphy). This pouch, known as the sali- 
vary syringe, has two pairs of muscles inserted on it. In the honey bee 
one pair arises on the hypopharyngeal region covering the oral surface of 
the prementum, but in Xylocopa these muscles (Fig. 163 C, Is) have 
migrated to the wall of the prementum. The other muscles (3s) arise 
ventrally in the prementum and are inserted on the sides of the syringe. 

The salivary syringe of Diptera (Fig. 172 D, Syr) and of Hemiptera 
(Fig. 179, Syr) is evidently also a derivative of the salivarium, though 
in these orders it has a terminal outlet duct (sm) that traverses the 
hypopharynx and opens on the tip of this organ. The ventral labial 
muscles are absent in both cases, but the dorsal dilators are present. In 
the Hemiptera the dilator muscles {dlsyr) arise on the inner faces of long 
basal plates of the hypopharynx (Fig. 180 B, hpl); in the Diptera they 
take their origin on the posterior wall of the sucking pump of the feeding 
apparatus (Fig. 172 D), but the pump chamber is evidently the cibarium, 
the floor of which is formed by the basal part of the hypopharynx. 

2. THE CEPHALIC STOMODAEM 

The stomodaeum in its generalized form is a simple tube extending 
from the mouth to the mesenteron (Fig. 189, Stom). In most insects, 
however, it is differentiated into several more or less distinct regions 
distinguished by variations in the diameter of the tube, accompanied by 
differences in the intima and in the muscular sheath (Fig. 190, Stom). 
The stomodaeal regions are structural adaptations to functional differ- 
ences in various sections of the tube, and they are not strictly homol- 
ogous in all insects. 



THE ORGANS OF INGESTION 


285 


The part of the stomodaeum contained in the head lies above the 
transverse bar of the tentorium and passes into the thorax tlu’ough 
the upper part of the foramen magnum (Fig. 155). It is embraced by the 
nerve connectives from the brain {Br) to the suboesophageal ganglion 
(SoeGng) ; the frontal ganglion {FrGng) lies on its dorsal wall anterior to 
the brain. The first part of the stomodaeum lies immediately within 
the mouth and may be termed the buccal cavity (BicC). Following the 
buccal cavity is the region of the 'pharynx (Phy), usually apparent as a 
dilatation of the stomodaeum between the frontal ganglion and the 
cerebral nerve connectives. Posterior to the brain the stomodaeum 
may take the form of a simple oesophageal tube, but in Orthoptera, 
Coleoptera, and some other insects it is here differentiated into a second 
pharyngeal region, or posterior pharynx (PPhy). The precerebral 
pharynx must then be distinguished as the anterior pharynx (Eidmann, 
1925). Following the posterior pharynx there may be an oesophagus 
(Qe), which generally enlarges into the crop, or ingluvies ((7r). 

All parts of the head stomodaeum, as well as the preoral epipharyngeal 
surface of the clypeus and labrum, are provided with dilator muscles 
arising on the head walls and on the tentorium (Fig. 155). The number 
of these muscles is not the same in all insects, but those that arise on the 
head wall maintain definite relations in their points of origin and insertion. 
They are therefore of much value for determining homologies both in 
the cranial areas of their attachments, and in the parts on which they are 
inserted. The dorsal series of these muscles is consistently di^^ded by 
the frontal ganglion connectives into an anterior set of muscles arising 
on the clypeus and labrum, and a posterior set arising on the frontal and 
parietal areas of the cranium. The following anterior and dorsal muscles 
are regularly present in orthopteroid insects, and representative muscles 
recur in most of the other orders. 

Compressores labri (Fig. 155, cplr). — A group of fibers within the 
labrum, attached on its anterior and posterior surfaces. 

Dilator es cibarii (dlcb). — A pair of muscles within the clypeus, arising 
on its anterior wall and inserted on the epipharyngeal surface of the 
cibarium. These muscles become the principal dilators of the sucking 
pump in D 3 discidae, Thysanoptera, Hemiptera, and Diptera. 

Dilatores buccales (dlbc). — A pair of muscles arising on the clypeus 
and inserted on the stomodaeum just within the mouth. 

The foregoing muscles lie anterior to the nerve connectives of the 
frontal ganglion {FrGng)] the following are inserted posterior to the 
connectives. 

Retr actor es angulorum oris {rao). — A pair of large muscles arising 
dorsally on the frons, inserted on the oral branches of the suspensorial 
sclerites of the hypopharynx. 



286 


PRINCIPLES OF INSECT MORPHOLOGY 


Dilatores pharyngis frontales (Idlphy). — One or more pairs of slender 
muscles arising on the irons, inserted on the anterior part of the pharynx. 

Dilatores pharyngis postfrontales (2dlphy). — One or more pairs of 
muscles arising on the postfrontal region of the cranium, inserted on the 
pharynx before the brain. 

Dilatores postpharyngeales (dlpphy). — One or more pairs of muscles 
arising on the vertex, inserted on the stomodaeum behind the brain. 

Besides these muscles there are also lateral and ventral dilators of the 
stomodaeum arising on the head walls and on the tentorium, but they are 
not so constant as the dorsal muscles, and their diagnostic value is less 
important. 

From the foregoing review we should note particularly the following 
points: (1) The muscles of the clypeus are distributed to the cibarium and 
to the buccal cavity; (2) the frontal ganglion lies over the anterior end of 
the pharynx, and its connectives go anterior to the retractor muscles of 
the mouth angles and the first pharyngeal dilators; (3) the dorsal dilators 
of the pharynx arise on the frontal and postfrontal regions of the cranium. 

3. THE FEEDING MECHANISM OF NEUROPTERA AND COLEOPTERA 

The feeding organs of Neuroptera and Coleoptera are in general of the 
orthopteroid type of structure, but in some members of each order they 
are specially modified for other purposes than those of biting and chewing, 
such as those of grasping, injecting, and sucking. In certain features, 
particularly in the structure of the labium, an interesting interrelation- 
ship is found between larval and adult forms. The labium of adult 
Coleoptera, for example, is a three-part structure resembling that of 
many Orthoptera in that the postlabium contains a distinct mentum 
and a submentum. In the Neuroptera, however, a true mentum is 
apparently never present, and some larval Coleoptera resemble Neurop- 
tera in the structure of the labium, while others have a labium like that 
of the adults of their own order. 

The Mandibles. — The jaws are the most important members of the 
feeding organs in biting insects, and in the Neuroptera and Coleoptera 
they usually preserve the orthopteroid structure. With phytophagous 
species there is generally a well-marked differentiation in each mandible 
between a distal incisor lobe -with cutting edges (Fig. 156 A, in) and a 
basal molar lobe {mol) provided with an irregular masticatory surface. 
In predacious species, however, the grasping function of the jaws is more 
important than that of chevung, and in such species the mandibles are 
usually simple biting organs vith strong incisor points (B, E), which 
may be notched or toothed, but in which effective molar surfaces are 
generally absent. In some forms with greatly enlarged jaws, as the male 
ctage beetles (Imcanidae), the huge mandibles have no function in connec- 



THE ORGANS OF INGESTION 


287 


tion with feeding and are used for holding the female at the time of 
mating. 

Among both the Neuroptera and the Coleoptera, predacious larvae 
of certain species feed only on the juices or liquefied body contents of their 
prey as they hold the latter in their jaws, and some of these larvae, by a 
special modification of the feeding mechanism, become true sucking 
insects. 

The most familiar insects having the grasping-sucking type of mouth 
parts are the larvae of the diving water beetles, Dytiscus and related 
genera. The mandibles of the Dytiscus larva are long, curved fangs 



Fig. 156. — Mandibles of Coleoptera and Neuroptera, and sucking apparatus of the 
larva of Dytiscus. A, mandibles of scarabaeid larva, posterior view. B, right mandible 
of adult Chrysopa. C, left mandible of Dytiscus larva, dorsal view, showing inner canal. 
D, section of head of Dytiscus larva showing cibarial (C6) and pharyngeal (Phy) pumps. 
{From Burgess, 1883.) E, mandibles of adult Plcrosticus, posterior view, showing grooves. 


(Fig. 156 C, Md) hinged to the anterior lateral angles of the head by 
dorsal and ventral articulations so that they work in a horizontal plane. 
Each mandible is traversed by a tubular canal (c), really a groove on 
the inner face of the appendage with confluent edges, opening distally 
near the tip (x) and proximally near the base of the jaw (y). The labrum 
(D, Lm) is sharply deflected against the base of the labium (Lb), where a 
marginal ridge of the former is securely held in a transverse groove of the 
labial surface. Just behind the closure thus formed is a transverse 
preoral chamber (Cb), which is evidently the cibarium (Fig. 155, Cb), its 
floor being the dorsal surface of the hypopharynx (Hphy). The lateral 
extremities of the cibarial chamber, or “mouth cavity,” extend to the 
bases of the mandibles, where, on each side, there is a small aperture to 
the exterior. When the mandibles are flexed, the proximal openings of 



288 


PRINCIPLES OF INSECT MORPHOLOGY 


their canals come into contact with the lateral apertures of the cibarium 
and thus establish continuous passages into the latter from the tips of the 
fangs. The dorsal wall of the cibarium is provided with strong dilator 
muscles (Fig. 156 D, dlch) arising on the clypeal region {Clp) of the 
frontoclypeal plate of the cranium (C). By the action of these muscles 
the closed cibarial chamber becomes a preoral pump. The true mouth of 
the Dytiscus larva (D, Mth) lies in the posterior wall of the cibarium. 
It leads into a large, strongly musculated anterior pharynx (Phy), which 
also apparently is a part of the pumping apparatus. 

When the Dytiscus larva closes its mandibles in the body of its prey, 
a poisonous and digestive fluid discharged from the stomach is ejected 
from the cibarium through the mandibular canals, which, as described by 
Blunck (1916a), spreads quickly through the body of the victim and 
rapidly dissolves the softer tissues. The liquefied material is then, by a 
reversal in the action of the pumping mechanism, sucked back into the 
pharynx and passed on to the stomach. Detailed descriptions of the 
feeding apparatus and the method of feeding of the Dytiscus larva are 
given by Burgess (1883), Rungius (1911), Blunck (1916a, 1918), Kor- 
schelt (1924), and Weber (1933). 

The occurrence of grooves on the mandibles is not unusual in Coleop- 
tera (Fig. 156 E, g ) ; and other predacious species have taken advantage 
of their presence in much the same way as has the Dytiscus larva. In the 
larvae of certain Lampyridae, for example, the mandibles are perforated 
by channels opening at their bases, through which a liquid is injected 
into the body of the prey. This liquid, according to Bugnion (1929a), 
comes from the stomach and converts the tissues of the recipient into a 
“bouillon nutritif”; but in the case of the lampyrids, Bugnion observes, 
ingestion takes place directly through the mouth and not by way of the 
mandibular canals. In some other lampyrid larvae the mandibles are 
simply grooved, but the grooves are converted into tubes by long acces- 
sory lobes applied against them. In still other species the accessory 
lobes are short and the mandibular grooves are open canals. The sucking 
apparatus of the lampyrid larvae, as illustrated by Bugnion (1929a, 
Fig. 21), appears to be principally, as in the Dytiscus larva, the cibarial 
chamber of the preoral cavity, with its dorsal dilator muscles arising on 
the clypeal area of the head wall. 

A grasping-sucking feeding mechanism occurs also in many predacious 
larvae of the Neuroptera that have long, fanglike jaws. The mandibles 
of such species are deeply grooved on their ventral surfaces, but here the 
closing lobes are long blades of the maxillae, which fit into the mandibular 
grooves and thus form tubular channels between the two appendages, 
through which the larva sucks out the juices of its victims. Familiar 
examples of neuropterous larvae thus equipped are the aphislions and 



THE ORGANS OF INGESTION 


289 


antiions. Lozinski (1908), in his study of the latter, describes a group of 
glandular cells in the wall of each maxillary blade, which discharge into 
a cuticular canal opening at the tip of the organ. The secretion from 
these cells, he believes, is poisonous and accounts for the ease with which 
the larva overcomes a struggling ant held in its fangs. 

The Maxillae. — It is seldom that any difficulty is encountered in a 
study of the maxilla in adult Neuroptera and Coleoptera, since the 
appendage usually preserves the typical generalized form and muscula- 
ture (Fig. 157 A, B). The posterior surfaces of both the cardo and stipes 
may be conspicuously marked by the lines of internal ridges, which give 



Fio. 157. — Maxillae of Coleoptera and Neuroptera. A, Chrysopa adult. B, Pterosticus 
adult. C, soarabaeid larva. D, carabid larva, Scarites. 

them a “divided” appearance, but each part preserves its unity, and the 
stipes may always be identified as such by the origin of the muscles of the 
palpi and terminal lobes within it (A). The galea sometimes appears to 
be two segmented, but the true galea is to be determined by the point of 
insertion of its flexor muscles (A, fga). In larval forms the maxilla often 
suffers a reduction, especially in its appendicular parts (C, D), and in 
such cases it is only by a comparative study of serially related species 
that the persisting lobes can be identified (see Blunck, 1918; Korschelt, 
1924). 

The Labium. — It is in the study of the labium that students of Neurop- 
tera and Coleoptera find themselves most often confronted with problems 
concerning the identities of the parts, and with difficulties in making 
satisfactory comparisons between divergent forms. Discrepancies of 
interpretation are in part merely the use of common terms in different 
senses by different writers, but in a larger measure the}'’ are the result of a 
failure to determine the fundamental morphology of the labium, which 
in most cases is readily disclosed by a study of the labial musculature. 

Neuroptera . — ^The parts of the labiiun in Neuroptera are likely to be 
misinterpreted because, in both larval and adult forms (Figs. 68, B, 82 B, 
158 B), the labium contains a middle plate (c), which at first sight appears 
to be a mental sclerite. An examination of the labial musculature, 



290 


PRINCIPLES OF INSECT MORPHOLOGY 


however, reveals that the median muscles {rst) are inserted on this plate, 
which is thus shown to belong to the prementum. The tentorial adductor 
muscles of the labium are inserted on the distal part of the prementum. 
The postmentum varies in size, but it consists of only one sclerite {Pmt). 
The labium of Neuroptera, therefore, is characterized by a differentiation 
of the premental sclerotization into a distal plate, or plates, bearing the 
insertions of the tentorial adductor muscles of the labium, and into a 
proximal plate giving insertion to the median retractor muscles. The 
postlabial sclerotization is not divided into a mentum and a submentum, 
as in adult Coleoptera, but there may be a wide membranous area distal 
to the single postlabial plate. 

The labium of an adult m 3 n'meleonid (Fig. 82 B) shows w’^ell the 
typical structure of the neuropterous labium. The postlabium contains a 
large proximal sclerite {Pmt), but its distal part is membranous and is 
traversed by the retractor muscles (rst) extending from the postmentum 
to the proximal sclerite (c) of the prementum. The sclerotization of the 
prementum is differentiated into a pair of anterior sclerites (ab) giving 
insertion to the tentorial adductors {adlb), and into the large proximal 
plate (c) on which the median retractors (rsi) are inserted. Each of the 
anterior sclerites (ah) is expanded on the lateral wall of the prementum in 
a triangular plate supporting the hypopharynx. The labium of Chrysopa 
(Fig. 158 B) is structurally the same as that of the myrmeleonid, but the 
postmental plate {Pint) is very long, and the proximal sclerite (c) of the 
prementum {Prmt) is a narrow transverse bar giving attachment to 
the retractor muscles {rst). The distal sclerotization of the prementum 
{ah), on which are inserted the tentorial adductors of the labium {adlb), is 
continuous with that of the broad spatulate ligula {Ldg). 

In the Sialidae the prementum is relatively large. In the larva of 
Conjdalus (Fig. 68 B, Prmt) its sclerotization includes a distal plate {ab) 
supporting the palpi and giving attachment to the tentorial adductor 
muscles, and a pair of proximal plates (c, c) on which the median muscles 
{rst) are inserted. The postmentum {Pmt) is broad but short and is 
continuous proximally with a well-developed gula {Gu) posterior to the 
tentorial pits {pt). 

Larval Coleoptera . — The labium of many coleopterous larvae has a 
structure A’^ery similar to that of the labium of larval and adult Neuroptera. 
In a silphid larva, for example (Fig. 158 A), the prementum {Prmt) con- 
tains two principal sclerites (a, c), on the proximal one of wliich (c) are 
inserted the median retractor muscles {rst), and on the distal one (a) 
the dorsal adductors. A pair of very small intermediate sclerites {b) is 
here present, however, which give insertion to the ventral adductors 
{2adlb). The proximal premental sclerite of coleopterous larvae (c) is 
commonlj' mistaken lor the mentum, but the attachment of the retractor 



THE ORGANS OF INGESTION 


291 


muscles {rst) on its base shows clearly that it is not the homologue of the 
mentum of an adult beetle (C, Mt)^ which always lies proximal to the 
insertions of the median muscles (D). The plate in question, on the other 
hand, corresponds exactly to the proximal premental sclerite in the labium 
of Chi'ijsopa (Fig. 15S B, c). The basal region of the silphid labium, lying 
proximal to the labial suture (A, lbs), contains a well-developed post- 
mental plate (S7}ii), which here evidently corresponds to the submentum 
of the adult, since there is a distinct though weakly sclerotized area 
distal to it (Mt) in the position of the mentum of an adult beetle (C, Mt). 
In many coleopterous larvae, however, the mentum is either entirely 



Fio, 158. — Various typos of labial structure. A, B, C, larva of Silpha, adult Chry- 
sopa, and adult Ptcrosliciia, showing corresponding di\’ision between promontum (Prmt) 
and postnjontum (Pmt) ns determined by the musculature. D, prementum and ligula of 
adult PlcToslicus. E, labium of larval dragonfly. F, labium of adult Bremus, lateral view. 

unrepresented, or its area is included in that of the single postmental 
plate. 

A simple condition of this type of structure in the coleopterous 
larval labium is shown in the Scarabaeidae (Fig. 159 A, B). The body 
of the labium here consists of a movable prementum (A, B, Prmt) having 
the hypopharynx (B, Hphy) adnate on its dorsal surface, and of a broad 
postmental plate (Pvit) in the ventral wall of the head. The median 
retractor muscles of the prementum (A, B, rst) arise on the proximal 
margin of the postmentum. The ventral wall of the prementum con- 
tains a distal sclerite (a) surrounding the bases of the palpi, two small 
intermediate sclerites (b) on which are inserted the ventral adductor 
muscles (B, 2adlb), and a large proximal plate (c) giving insertion to the 
retractor muscles (rst). The proximal plate is reflected dorsally on the 



292 


PRINCIPLES OF INSECT MORPHOLOGY 


sides of the prementum to the base of the irregular hypopharynx (B, C, 
Hphy). Various other coelopterous larvae are found to have this same 
type of structure in the labium, but an extensive comparative study of the 
labial musculature will be necessary to determine its prevalence. The 
median muscles of the labium (rst) function as retractors of the pre- 
mentum when the prementum and postmentum are separated by a 
membranous area, but if the adjacent plates are hinged to each other the 
muscles become flexors (adductors) of the prementum. The second 
function is well exemplifled in the larva of Dermestes. 

A second type of labial structure, which is identical with that of the 
adult, also occurs in the larvae of Coleoptera. In the melandryid larva 



Fio. 159. — Labium of Coleoptera. A, ecarabaeid larva, Ochrosidia villosa, ventral 
view. B, same, labium and hypopharynx, lateral view. C, same, hypopharynx and 
mouth, dorsal \'iew. D, labium of adult Phyllophaga^ showing submentum invaginated 
between mentum and gula. E, same, longitudinal section. 

(Fig. 160 A) the middle part of the labium {Mt), though weakly sclero- 
tized, is a rigid extension from the submentum {Sml) and has no muscles 
inserted upon it. This part of the lanml labium evidently becomes the 
mentum of the adult (B, Mi). The prementum is the small terminal part 
of the labium (A, Prmt) retractile within the mentum. The same type of 
structure occurs likewise in some larval Staphylinidae (C, D), in which 
the mental region (Mt) may be largely membranous, but it is the area 
of the labium containing the mentum of the adult beetle (Fig. 68 A). 
In carabid larvae (Fig. 67 D) the labial plate lying between the maxillary 
cardines is a part of the postmentum (Pmt), and apparently its distal 
part is separated in the adult (Fig. 158 C) from the proximal submental 
area to form the mentum (Mt). 

A gular plate is either present or absent in the larvae of Coleoptera 
and when present is variously developed. Frequently, however, the gular 
region is entirely membranous (Fig. 160 A, gu), and it is often almost 



THE ORGANS OF INGESTION 


293 


obliterated by an approximation of the postgenal areas of the cranium, 
being reduced in such cases to a median membranous line, or “suture,” 
proximal to the tentorial pits (D, gu). In certain coleopterous larvae, 
especially in the Prionidae, the base of the labium is separated from the 
neck by a sclerotization uniting the postgenal areas of the cranium, which 
appears to be a hypostomal bridge. In these lar^rae there is no true gula, 
since the tentorial pits remain at the posterior margin of the head. 

Associated with the labium in many coleopterous larvae is a pair of 
bars extending outward from the sides of the prementum or the hypo- 
pharynx to the posterior articulations of the mandibles or to the distal 
extremities of the hjrpostomal margins of the cranium (Fig. 67 B, d). 
These bars, often called “bracons,” lie in the membranous ventral wall of 
the head between the bases of the mandibles and the maxillae. 

AduU Coleoptera . — ^The labium of adult Coleoptera is typically 
a three-part structure (Fig. 158 C), there being in its ventral wall a well- 
defined middle plate {Mt) which lies proximal to the insertions of all 
the labial muscles and is, therefore, a true mentum, that is, a distal plate 
of the postlabium. The prementum (Prmt) is usually small, and its 
sclerotization is variable, but it always bears the insertions of all the 
stipital muscles of the labium (D, stmcls). Generally the prementum is 
retractile into the mental region, since the base of the prementum is 
usually attached to the mentum by an infolded membrane (C, D, /); 
but in some cases the prementum is hinged to the distal margin of the 
mentum, and its movement is then one of flexion on the latter (Fig. 
159 E). 

The ligula is generally a distinct part of the adult coleopterous 
labium, uath the glossae and paraglossae more or less separated (Fig. 
158 D, Gl, Pgl), though the glossae are usually united in a median lobe; 
but the entire ligula may be a single broad terminal flap between the 
palpi (Fig. 169 D, Lig). The muscles of the ligula, when present, as 
well as those of the palpi arise in the prementum (Fig. 158 D). 

The basal region of the adult coleopterous labium generally contains a 
mentum and a submentum (Fig. 158 C, Mt, Smt), though the respective 
areas of the two plates are sometimes confluent or are separated only by a 
groove or a transverse depression (Fig. 67 C) . The wide anterior part of 
the submentum lies between the maxillary cardines (Figs. 67 C, 68 A) ; 
proximally the submentum extends to the posterior tentorial pits (pt), and 
its length, therefore, varies according to the position of the pits. Proximal 
to the pits it is continuous with the gula (Gu). The mentum projects 
forward from the distal margin of the submentum between the bases of 
the maxillary stipites and supports the prementum. It is usually a well- 
developed plate, but its size is variable (Figs. 67 C, 68 A, 158 C, 160 B, 
Mt). When the entire postlabium is sclerotized in the larva, the mentum 



294 


PRINCIPLES OF INSECT MORPHOLOGY 


and submentum of the adult are to be regarded as subdivisions of the 
postmental plate; if only the proximal part of the larval postlabium is 
sclerotized, the mentum appears to be developed in the distal membranous 
part. 

An unusual labial structure is found in some adult Scarabaeidae, 
as in Phyllophaga (Fig. 159 D), in which the labium appears to consist 
onl}'' of a prementum (Prmt) and a mentum (Mt), projecting beyond the 



Fig. ICO. — Head and mouth parts of Coleoptera. A, larva of Melandrya with mem- 
hranous gular region (pu). B, adult M striata with well-developed gula (Gu). C, larva 
of Staphylinus wnth gular area represented by a median suture (pu). D, adult staphylinid, 
Thinopinus picius, with same type of structure, but retaining hypostomal sutures (hs). 
Cv, neck; Gu, gula; pu, gular area, either wide or reduced to a median “suture”: hi, hypo- 
stomal lobe; hs, hypostomal suture; Hst, hypostoma. Poc, postocciput; PoR, postoccipital 
ridge: pos, postoccipital suture; ps, pleurostomal suture 


large gula {Gu). An examination of the inner surface of the labium (E), 
however, shows that the submentum is represented by an internal 
recurved plate {Smt) deeply inflected between the mentum and the gula, 
on which arise the median muscles of the prementum (D, E, rst). These 
muscles here act as flexors of the prementum, since the prementum has a 
definite hinge on the distal margin of the mentum. 



THE ORGANS OF INGESTION 


295 


4. THE FEEDING MECHANISM OF HYMENOPTERA 

The Hymenoptera in classification are usually assigned a place near 
the top of the series of insect orders, but their structural attainments 
seem scarcely to warrant so liigh a rank, and the hymenopterous mouth 
parts, though adapted in the adult stage for feeding on liquids, are never 
so highly modified to this end as are the organs of Lepidoptera, Diptera, 
and Hemiptera. The essential features in the sucking mechanism of the 
higher hymenopterous families are present likewise in the lower members 
of the group, shoving that the basic structure formed in the latter has 
been evolved into the specialized structure of the former, and suggesting 
also that the fundamental mechanism of the mouth parts must have been 
acquired in the first place as an adaptation to the feeding habits of 
ancestral forms resembling the modem sawflies, or members of the chalas- 
togastrous families. 

The Larval Mouth Parts. — ^The mouth parts of all hymenopterous 
larvae are in some respects degenerate, and in parasitic species the 
reduction is usually carried much further than in others; but in all forms 
the basic structure of the mouth parts is the same. The feature char- 
acteristic of them is a close association or union of the maxillae, 
the labium, and the hypopharynx to form an under-hp complex, in which 
the ligula and the hypopharynx are combined in a median lobe on which 
opens the duct of the labial glands. These glands, at least in the mature 
stage of the larva, produce the silklike material from which the fabric 
of the cocoon is spun. The composite feeding and spinning organ of 
hymenopterous larvae is in many respects identical vith the similar 
organ of lepidopterous larvae, and the likeness in the mouth parts only 
accentuates the general resemblance between the larvae of the two groups, 
so conspicuous in the body form of a sawfly larva and a caterpillar. 

The hymenopterous larval mouth parts preserve a more generalized 
condition in the chalastogastrous families, as shown in the larva of Cimbex 
(Fig. 161 A, B, C). The mandibles (A) are strong biting jaws of the 
ordinary type of structure. The maxillae are united basally with the 
labium (B), but each consists of a cardo and a stipes (C), ■with two 
terminal lobes and a segmented palpus. The labium is distinctly divided 
into a wide, membranous postmentum (B, Pmt) and a distal prementum 
(Print) bearing a pair of palpi (LbPlp). The ligula (Ldg) and the hypo- 
pharynx (Hphy) are united in a median distal lobe, on the apex of which 
is the spinneret (Sr) containing the orifice of the labial glands. 

In the aculeate Hymenoptera the larval mouth parts become more 
simplified, and in some respects more specialized. In Apidae and 
Vespidae (Fig. 161 D) the mandibles retain the ordinary form and 
position, but the maxillae and labium are reduced. Each maxilla may 



296 


PRINCIPLES OF INSECT MORPHOLOGY 


consist of a distinct cardo and stipes {Cd, St), but it always terminates in a 
single small lobe, which, by comparison with Cimbex (C), is apparently 
the galea, at the base of which is a small papilla possibly representing the 
palpus. The labium is a simple structure, composed of a basal postmen- 
tum (Pmt) and a distal prementum (Prmt), but its wall contains no 
distinct sclerites. 



It is in parasitic species that the mouth parts of larval Hymenoptera 
become most specialized and acquire a distinctive structure. The weakly 
sclerotized head capsule is generally strengthened along its subgenal 
regions by strong marginal ridges (Fig. 161 E), forming a prominent 
bar on each side, differentiated into a hypostoma (Hst) supporting the 
labium and maxillae, and a pleurostoma (Plst) bearing the mandibular 





THE ORGANS OF INGESTION 


297 


articulations. Anterior to the mandibles the lateral ridges are usuallj’- 
produced as a pair of epistomal bars (E, F, G, Est) extending to the ante- 
rior tentorial pits {at), but generally the bars are not connected between 
the pits, the frons and clsrpeus being thus continuous (F, FrClp). In 
some cases the epistomal bars are united by a transverse ridge at the base 
of the labrum (H). 

The mandibles of more generahzed parasitic species have the usual 
form and position (Fig. 161 E, Md); but very commonly they assume 
a horizontal position and become more or less concealed behind the 
labrum (F, G, H). In some forms they are very small or rudimentary. 

The maxillae in most parasitic species are simple elongate 
lobes with no demarkation into cardo and stipes (Fig. 161 E, Mx). 
Very commonly, however, a sclerotic spur (s) from the h 3 rpostoma just 
behind the mandible extends into the wall of the maxilla, and it may 
completely divide the maxilla into a proximal part, united vdth the 
postmentum, and a free distal part (F, G, Mx) . Generally the posterior 
(or ventral) margin of the maxilla is reinforced by a sclerotic bar (g), 
which is sometimes united proximally with the hypostoma (E), but 
which more commonly ends in a free basal expansion (F, G), though it 
may be more or less reduced or almost completely suppressed (H, q). 

The labium preserves its division into postmentum and prementum 
(Fig. 161 E, F, Pmt, Prmt). The first is always membranous, but the 
prementum usually contains a marginal sclerotization, with sometimes 
a central expansion. The shape of the premental sclerite is highly 
variable (F, G, H, t), but its general form is often characteristic of genera 
or groups of genera. In some species the premental sclerite articulates 
laterally with the distal extremities of the maxillary sclerites (G, r), on 
which the prementum apparently is movable. The structure is then 
very similar to that characteristic of the spinning apparatus of lepi- 
dopterous larvae (Fig. 165 D, E). A curious conformation in the mouth 
region sometimes results from a suppression of the hypostomal ridges 
and the proximal parts of the maxillary sclerites, accompanied by a 
strong development of the pleurostomal and epistomal ridges and the 
spurs (s) of the maxiUary sclerites to form an oral framework (H) support- 
ing the labrum, the mandibles, the maxillary lobes, and the prementum. 

The Feeding Mechanism of Adult Hymenoptera. — The mouth parts of 
adult Hymenoptera have the same fundamental characteristic as those of 
the larva, namely, the union of the maxillae with the labium, but the 
terminal parts of the appendages are better developed in the imago and 
are readily identified in the more generahzed forms, while the secretion 
of the labial glands has the usual “salivary” function. 

The Mouth Parts of a Sawfly . — ^The basic structure of the mouth 
parts of adult Hymenoptera is well shown in the Tenthredinidae. The 



298 


PRINCIPLES OF INSECT MORPHOLOGY 


mandibles here have the form of typical biting jaws (Fig. 162 B). The 
maxillae and labium are united in a composite structure (A) suspended 
from the postgenal margins of the cranium by the basal articulations 
of the maxillary cardines. Each maxilla (C) is composed of a triangular 
cardo {Cd) and an elongate stipes {St ) ; the stipes bears a five-segmented 
palpus, a broad galea (Ga), and a small lacinia (Ac). The labium is 
somewhat compressed between the maxillary stipites and cardines (A), 
to which it is attached by membrane. The body of the labium consists 
of a sclerotized prementum {Prmi) and of a large, mostly membranous 
postmentum {Pmt) containing a small proximal sclerite {Smt). The 
prementum {Prmt) is reinforced by a median internal ridge continued 
forward from a thickening of its posterior margin and thus appears 
to be composed of a pair of united sclerites. The muscles of the palpi 



Fig. 162. — Mouth parts of adult Hymenoptera. A, Pleronidea ribesii, labium and 
maxillae. B, same, mandibles. C, same, maxilla detached. D, Andrena carlini, labium 
and maxillae, posterior \dow. E, same, lateral view. F, mandible of Andrena. 

arise on its inner surface, and the cranial muscles of the labium are 
inserted upon it. The proximal sclerite of the postmentum {Smt) has 
the position of a submentum and apparently is to be identified with the 
lorum of the Apidae (Figs. 162 D, 163 A, Lr), since it has the relation of 
the lorum in the basal mechanism of the maxillolabial complex, though 
it is not connected with the cardines. The terminal lobes of the ten- 
thredinid labium include a pair of free lateral paraglossae (Fig. 162 A, 
Pgl) and a narrow median lobe {Gl), which is evidently the united glossae. 
Arising from the dorsal surface of the labium is a median elevation which 
is probably the hypopharynx. 

The maxillolabial complex of Tenthredinidae is attached to the 
posterior wall of the head between the postgenal margins of the epi- 
cranium by ample membranes, which allow it a free movement on the 
suspensoria formed of the maxillary cardines. A line of Hexure crosses 
ihe posterior part of the organ through the stipitocardinal sutures of the 
maxillae laterally, curving anterior to the submental plate of the labium. 


THE ORGANS OF INGESTION 


299 


In the usual position^ the part distal to this fold lies parallel with the 
under surface of the head, while the cardosubmental part is bent abruptly 
toward the head, where it is attached. The entire organ can thus be 
extended by swinging distally on the maxillary cardines. The distal 
parts of the maxillae lie dorsal (anterior) to the bases of the labial palpi, 
and the maxillary lobes are turned in a plane vertical to the surface of 
the ligula, so that the terminal parts of the labium and maxillae form the 
floor and sides of a wide troughlike channel leading upward to the mouth 
over the dorsal surface of the labium. 

The maxillolabial organ thus simply formed in the Tenthredinidae 
from the usual parts of the maxillary and labial appendages is retained 
with but slight modifications in the majority of adult Hymenoptera, as 
shown in the series of studies by Bugnion (1925, 1927, 1929, 1930), and 
it furnishes the basis of the more specialized lapping and sucking appa- 
ratus of the bees. The structure and mechanism of the mouth parts of 
Sphecidae are elaborately described by Ulrich (1924). 

The Feeding Mechanism of Bees. — In the bees the mandibles lose 
the typical biting form and become more or less flattened or spoon 
shaped to form tools that may be used for a variety of purposes (Fig. 
162 F). The maxillolabial complex is lengthened (D, E), and its free 
parts become modified as accessories to a sucking pump developed from 
the buccopharyngeal region of the stomodaeum. 

A generalized condition of the apoid type of mouth parts is found 
in some of the solitary bees, such as Andrena (Fig. 162 D, E). The 
prementum (Prmi) is here elongate, and the median glossal lobe (Gl) 
forms a short, conical, hairy “tongue,” with the paraglossae {Pgl) 
diverging from its base. In the maxillae the laciniae are lost, but the 
galeae (Ga) are enlarged; the palpi {MxPlp) are reduced in size, though 
they retain a distinct segmentation. The cardines (Cd) are long, rod- 
like suspensoria of the maxillolabial complex, articulated with the 
postgenae, which are united in a median hypostomal bridge behind the 
base of the labium, as in the honey bee (Fig. 65 C). Between the cardines 
is a small V-shaped sclerite (Fig. 162 D, Lr) articulating laterally with 
the distal ends of the cardines and supporting the base of the labium. 
This sclerite, known as the lorum, lies proximal to the transverse line of 
flexion passing through the stipitocardinal joints and thus corresponds 
in position to the submental sclerite of Pieronidea (A, Smt). A small 
triangular plate in Andrena united ndth the base of the prementum 
(D, Mt) is possibly a mentum. 

In the higher bees, such as Xylocopa, Bombus, and Apis, still further 
modifications have taken place in the maxillolabial apparatus. The 
galeae are large flat blades (Fig. 163 A, Ga) much longer than the maxil- 
lary stipites; the laciniae are rudimentary or absent; the maxillary 



300 


PRINCIPLES OF INSECT MORPHOLOGY 


palpi, though long in Xylocopa, are reduced to small pegs in Apis (A, 
MxPlp). The glossal tongue of the labium (Gl) is greatly lengthened 
and highly mobile, being flexible in all directions and capable of an active 
movement of protraction and retraction. Its base is closely embraced 
by the relatively small paraglossae (Pgl)- The labial palpi are long and 



Fio. 163. — Feeding mechanism of adult bees. A, Apis unicolor, worker, labium and 
maxillae ■\\ith attachment to head. B, section of head of Xylocopa virginica, showing 
sucking pump {Pmp). C, section of basal part of labium of Xylocopa, showing salivary 
syringe {Syr) and muscles of the prcmentum, 

four segmented (LbPlp), the two basal segments of each forming a 
large flat blade tapering distally to the two small distal segments. The 
region of the long labial stipites, or premen turn (Prmt), contains a 
large, strongly sclerotized plate (commonly called the “mentum” by 
students of Hymenoptera). The membranous palpigers and the base 
of the ligula are partly retractile into the anterior end of the stipital 
region. At the base of the prementum is a small triangular sclerite 





THE ORGANS OF INGESTION 


301 


(Mt), which appears to belong to the postlabium, since it lies proximal 
to the insertions of the posterior cranial muscles of the labium (C, 2adlb), 
and may therefore be termed the mentum. The apex of this sclerite fits 
into the concave angle of the lorum (A, Lr), which, as in Andrena (Fig. 
162 D), articulates by its lateral extremities with the distal ends of the 
maxillary cardines. Retractor muscles of the prementiun are absent 
in Hymenoptera. 

The long glossal tongue of the bees is an organ of particular importance 
in the feeding mechanism. It is densely clothed with hairs except at its 
base and terminates in a small lobe called the “spoon,” or flabellum 
(Fig. 163 A, FbT). When the bee feeds on a liquid easily accessible, the 
broad maxillary galeae and the labial palpi are brought together over 
the tongue, thus improvising a tubular proboscis, the end of which is 
thrust into the food liquid. By a rapid back-and-forth movement 
of the tongue the liquid is drawn up into the tube, and from the latter 
it is sucked up to the mouth by the stomodaeal pump. If the food liquid 
is confined in a narrow space, as in the corolla of a flower, however, the 
tongue may be thrust out far beyond the ends of the maxillae in order 
to obtain it. Both the tongue and the paraglossae are deeply retractile 
into the distal part of the prementum by an infolding of the membrane 
at their bases caused by a contraction of the glossal muscles (C, fgl). In 
some of the shorter tongued bees the maxillary galeae are stiff, sharp- 
pointed blades and are used for cutting through the outer wall of a corolla 
in order to gain access to the nectar within. 

The posterior surface of the glossal tongue is excavated by a deep 
channel which extends from the flabellum to the base of the tongue, 
where the latter is closely embraced by the paraglossal lobes. Dorsally, 
the paraglossae cover the salivary orifice located on the oral surface 
of the labium at the distal end of the prementum (Fig. 163 C, SIO). The 
paraglossae thus evidently serve to conduct the salivary secretion around 
the base of the tongue into the channel on the ventral side of the organ, 
through which it is conveyed to the tip of the latter to be mixed with 
the food during ingestion. 

The salivary duct (Fig. 163 C, SID) opens into the lumen of an 
expulsive apparatus known as the salivary syringe {Syr). This organ 
consists of an elongate pouch with two pairs of muscles inserted on 
its walls. Its outlet is the functional sahvary orifice {SIO) located 
on the distal extremity of the prementum between the base of the glossal 
tongue and the rudimentary hypopharynx {Hphy). There can be no 
doubt, therefore, that the syringe is a development of the salivarium, 
which in its primitive form is a simple salivary pocket at the junction of 
the hypopharynx with the oral surface of the labium (Fig. 155, Slv). In 
Xylocopa the syringe is provided with two pairs of long muscles arising 



302 


PRINCIPLES OF INSECT MORPHOLOGY 


in the prementum (Fig. 163 C, Is, 3s), but in Apis the dorsal pair (Is) 
have their origin on the lateral margins of the hypopharyngeal surface 
{Hphy) and thus correspond to the usual hypopharyngeal dilators of 
the salivarium (Fig. 155, Is). 

The sucking pump of the bees is a large muscular sac lying entirely 
within the head anterior to the brain (Fig. 163 B, Pmp). The morphology 
of the organ is not entirely clear, but, judging from its musculature, it 
includes without doubt the pharynx and the buccal cavity and perhaps 
the cibarium. Its dorsal dilator muscles are separated into two groups 
inserted anterior and posterior to the frontal ganglion {FrGng) and its 
connectives. Those of the first group (dlbc) arise on the clypeus; those 
of the second group (dlphy) take their origin on the frons. On the floor 
of the pump, just within the mouth, is a broad sclerotic plate, from which 
a long arm {h) extends posteriorly and dorsally on each side in the lateral 
wall of the organ and gives insertion to a pair of convergent muscles, 
one arising on the clypeus, the other on the frons. It is perhaps possible 
that these bars represent the oral arms of the hypopharyngeal suspen- 
soria of more generalized insects (Fig. 60, y). A pair of large ventral 
dilator muscles of the pump arise on the transverse bar of the tentorium. 
The posterior end of the pump narrows to a more slender tube, which 
passes between the brain and the suboesophageal ganglion, and enlarges 
again in a small posterior pharynx (PPhy) lying in the rear part of the 
head behind the brain. The posterior pharynx is provided with a pair of 
long slender dorsal dilator muscles (dlpphy), and a pair of short ventral 
dilators arising on the tentorium. 

6. THE FEEDING MECHANISM OF LEPIDOPTERA 

In the Lepidoptera the structural divergence between the mouth 
parts of the larva and those of the adult has been carried to a still greater 
degree than in the Hymenoptera. The mouth parts of a caterpillar show 
a general resemblance to those of the larvae of Hymenoptera, and in 
each group the salivary glands secrete a substance that becomes silky 
when extruded and which may be used for constructing a cocoon. 
The mouth parts of a moth or butterfly, on the other hand, have little 
to suggest a common origin wdth the mouth parts of a wasp or bee, since 
in the majoritj’’ of Lepidoptera the mandibles are rudimentary or absent 
and the labium is eliminated from the feeding mechanism. In all but 
certain generalized forms the maxillae remain as the only appendages 
involved in the apparatus of ingestion, and they are greatly modified by a 
reduction of the palpi, the loss of the laciniae, and the elongation of the 
galeae to form a tubular proboscis. The sucking pump of the Lepidoptera, 
as in the Hymenoptera, is formed largely of the precerebral pharyngeal 
region of tiie stomodaeum, but it appears to include the cibarium. Its 



THE ORGANS OF INGESTION 


303 


dorsal dilator muscles take their origin on the frontoclypeal plate of 
the head wall. 

The Feeding and Spinning Organs of a Caterpillar 

The mouth parts of a caterpillar are of the biting and chewing type of 
structure as far as the function of feeding is concerned, but they are used 
for various purposes other than that of taking food. The mandibles, 
for example, serve with many species as implements for gnawing and 
tunneling, while the maxillae, labium, and hypopharjmx are always 
united in a large under-lip complex on which opens the duct of the silk- 
forming labial glands, and its activities, therefore, mostly pertain to 
the function of “spinning.” 

The Mandibles. — The caterpillar’s mandibles are jaws of the ordinary 
biting and chewing form (Fig. 164 B). Each is hinged to the head by 
posterior and anterior articulations (o’, c) of the usual type of structure. 
The abductor muscles are relatively small, but the great adductors (Fig. 
64 B, admd) occupy most of the lateral parts of the head cavity and 
appear to determine the size and form of the lateral hemispheres of the 
cranium. Some species of caterpillars are provided with a pair of large 
tubular mandibular glands reaching often far back into the thorax and 
abdomen, the duct of each extending down to the inner edge of the base 
of the mandible close to the apodeme of the adductor muscle. 

The MaxiIlolabial-h 3 T)opharyngeal Complex. — The maxillae, the 
labium, and the hypopharynx in the caterpillars, as in h 3 Tnenopterous 
larvae, are united to form a large composite structure that projects like 
a thick under lip behind the mouth and bears the spinneret at its tip 
(Fig. 164 C). Basally the organ is supported on the hypostomal lobes 
{Hst, Hst) of the postgenae, which are approximated medially between 
the neck and the base of the labium. 

The walls of the under-lip complex (Fig. 164 C) may be largely 
membranous, the sclerotization being usually broken up into various 
small plates; but by observing certain landmarks the components of 
the organ can be pretty well defined. Two lateral lobes, representing 
the maxillae, are more or less distinct from a median lobe formed of the 
labium and hypopharynx. A small plate {Cd) at the base of each 
lateral lobe is evidently the cardo, since it articulates (o”) with the 
hypostoma {Hst) and bears the insertions of the tentorial cardinal 
muscles. On the mesal border of each lateral lobe is the line of a 
strong internal ridge (Fig. 164 C, q) upon Avhich are inserted the usual 
stipital muscles of the maxilla. The areas laterad of the ridges, therefore, 
are the maxillary stipites {St). Each stipital area ends distally in a 
membranous lobe (Lo), usually having small sclerites in its walls and 
bearing terminal papillae provided with sense organs. Three muscles 



304 


PRINCIPLES OF INSECT MORPHOLOGY 


are inserted on the base of the lobe, two arising in the stipes and one on 
the hypostoma, but the homology of the lobe is difficult to determine in 
the ordinary caterpillar structure. In the micropterygid Sabatinca. 
however, the larval maxilla, as shown by Tillyard (1922), ends with a 
three-segmented palpus and a distinct lacinia and galea (Fig. 167 A). 

The median component of the under-lip complex of the caterpillar 
consists of the labium and the hypopharynx. Its proximal part is the 
postmentum (Fig. 164 C, Pmt), which may be an entirely membranous 
area, though it frequently contains a postmental sclerite (Fig. 164 



Fio. 164. — Mouth parts of larval Lepidoptera. A, Lycophotia margaritosa, labrum, 
anterior view. B, Ealigmene acrca, mandibles, ventral ^^ew. C, same, labium, hypo- 
pharynx, and maxillae united and suspended from hypostomal lobes of cranium. 

C, pmi). The distal part, which forms a free median lobe between 
the terminal lobes of the maxillae (Lo), is the prementum {Prmt) with 
the hypopharynx {Hphy) adnate upon its anterior surface. It bears 
distally the spinneret (Sr), a hollow spine having the orifice of the silk 
duct at its extremity. Labial palpi are absent, unless they are repre- 
sented by the pair of small papillae located at the sides of the spinneret. 

The mouth parts of larval Trichoptera are structurally identical vdth 
those of the caterpillars. The maxillae and labium are united in the 
same manner, and the hypopharynx is fused with the small prementum. 
The postmentum, however, is elongate and contains a relatively large 
plate included between the elongate postgenal areas of the cranium. 
In some species the postmental plate simulates a gula, but the posterior 
tentorial pits are always at the neck margin of the head. All the labial 
muscles are inserted on the prementum, but the median retractors are 
absent, as they are in the caterpillars. The maxillary musculature 
is the same as that of a caterpillar. 

The Spinning Apparatus. — The material of the silk threads spun by 
caterpillars is secreted by the labial glands, which consist of a pair of 
greatly elongate tubes lying in the body cavity at the sides of the ali- 
mentary canal (Fig. 196, SkGl). The ducts of these silk glands, after 


THE ORGANS OF INGESTION 


305 


receiving the ducts of a pair of small accessory acinous glands, sometimes 
called the glands of Filippi, unite in a short median conduit that opens 
into the base of an organ known as the silk press (Fig. 165, A, B, E, Pr). 
The press lies in the median lobe of the mouth parts formed of the united 
hypopharynx and prementum (A, C). It discharges through a narrow 
terminal duct that opens on the tip of the spinneret (Sr) located on the 
distal surface of the hypopharyngeal-premental lobe. The dorsal wall 
of the press is deeply invaginated into the lumen of the organ (F) and 
contains a median sclerotic bar, or raphe (Rph), on which are inserted 



Fig. 165. — Spinning apparatus of caterpillars. A, premento-hypopbaryngeal lobe 
of a nootuid caterpillar, with silk press and spinneret, lateral view. B, same, ventral view. 
C, same of Malacosoma americana, lateral view. D, premento-hypopharjmgeal lobe of a 
nootuid, showing support on maxillary arms (g), and muscles, lateral view. E, same in 
cross-section, posterior view. F, diagrammatic cross-section of silk press. 

two or three pairs of muscles (A, B, C, E, F, 17, 18) having their origins 
on the dorsolateral parts of the spinneret-bearing lobe. Another pair of 
muscles (19) arising ventrally in the premental part of the lobe is inserted 
on the lateral walls of the press (E, F). Both sets of muscles apparently 
are dilators of the press lumen, the antagonistic force being the elasticity 
of the infolded dorsal wall of the organ. 

Morphologically the silk press of the caterpillars is a highly specialized 
development of the salivarium of more generalized insects (Fig. 155, Slv). 
By the complete union of the hypopharynx with the prementum the 
press has become entirely enclosed between the component elements 
of the hypopharyngeal-premental lobe, and its outlet duct represents the 
persisting remnant of the salivary passage (sm) between the hypopharynx 
and the labium. Its dorsal muscles are the hypopharyngeal dilators 



306 


PRINCIPLES OF INSECT MORPHOLOGY 


of the salivarium (Is), its ventral muscles are those normally arising in 
the prementum (2s, 3s). 

The spinneret-bearing lobe of the caterpillar is supported laterally 
at its base on two sclerotic bars (Fig. 165 D, E, ?), which are the distal 
arms of the mesal ridges of the maxillary stipites (Fig. 164 C, q) articu- 
lated by their extremities (Fig. 165 D, E, r) with the sides of the premen- 
tum. Upon these fulcra the entire spinning apparatus can be swung up 
and down, or anteriorly and posteriorly, by muscles inserted upon its 
base. The spinning muscles comprise a pair of ventral labial muscles {15) 
inserted on the base of the prementum, and a pair of dorsal hypopharyn- 
geal muscles {16) inserted on the base of the hypopharyngeal surface, 
both pairs taking their origin on the tentorium. The fulcral arms (g) 
give insertion to a pair of maxillary adductors {11). The numerous 

m 1 



Flo. 166. — Anterior part of the stomodaeum of a noctuid caterpillar. 20-23, dilator 
muscles of buccal ca^’ity arising on clypeal triangle of cranium; 2i-27, precerebral dorsal 
dilators of pharyn.\; 28-30, postcerebral dorsal dilators of oesophagus; 31-36, ventral 
dilators of pharynx and oesophagus. 

other movements made by the caterpillar in the act of spinning are 
produced by the elaborate musculature of the back of the head and the 
anterior part of the body. 

The Head Stomodaeum. — ^The mouth of the caterpillar opens into 
a wide stomodaeal chamber, covered externally by broad plaques of 
muscle fibers, bung anterior to the nerve ring of the head (Fig. 166, 
BnC, Phy). The chamber is evidently the buccopharyngeal region of 
the stomodaeum, since its dorsal dilator muscles are separated into two 
groups by the frontal ganglion and its connectives, those of one group 
{22, 23) arising on the triangular clypeus, while those of the other group 
{24, 25, 26, 27) arise on the postclypeal areas of the head. A third 
anterior set of muscles {20, 21), arising also on the clypeus, is inserted 
just before the first transverse muscles (a) of the stomodaeal wall. These 
muscles are clearly cibarial muscles. The part of the stomodaeum lying 



TRE ORGANS OF INGESTION 


307 


in the head behind the nerve ring is merely a wide cylindrical oesophagus 
(Oe) with strong circular muscles. The dilator muscles inserted upon it, 
however, show that this part of the stomodaeum in the caterpillar corre- 
sponds to the posterior pharynx inOrthoptera and Coleoptera (Fig. 156 D 
192, 193, PPhy). 

The Feeding Organs op Adult Lepidoptera 

With most of the Lepidoptera the mouth parts undergo a radical 
change in structure during the metamorphosis from the larva to the 
imago. The mandibles become rudimentary or are entirely suppressed; 



Fig. 167. — Mouth parts of Micropterygidae. A, Sabatinca barbarica, larval maxilla. 
B, Micropteryx ammanella, head of adult. C, Micropteryx aruncella, maxilla of adult. 
D, Mnemonica auricyanea, maxilla of adult. E, same, labium of adult, F, same, head 
of pupa. G, Sabatinca incongruella, mandibles of pupa. (A, C, G Srom Tillyard, 1922, 
1923; B, D, E, F from Busck and Boving, 1914.) 

the terminal parts of the maxillae are transformed into long half tubes, 
which, together, form the characteristic coiled proboscis of the imago (Fig. 
168, Prh ) ; the labium is reduced to little more than a flap, but it acquires 
a pair of large palpi (LbPlp); the anterior part of the stomodaeum is 
developed into an efficient sucking pump (Fig. 169 F, Pmp). In the 
primitive family Micropterygidae the adult mouth parts have a more 
generalized structure, and one which clearly demonstrates the evolution 
of the sucking apparatus of the Lepidoptera from mouth parts of the 
usual biting type, and not from a mouth-part structure of the larval t3q3e. 
The feeding and spinning mouth parts of the caterpillar, therefore, appear 
to represent a specialized larval condition adaptive to the needs of the 
caterpillar. 

Generalized Lepidopterous Mouth Parts. — The members of the 
Micropterygidae are moths in every essential respect, but they have 
mandibulate mouth parts in the larval, pupal, and adult stages. The 
mandibles of the pupa and imago are typical functional jaws (Fig. 167 



308 


PRINCIPLES OF INSECT MORPHOLOGY 


B, G). Those of the adult are said by Tdlyard (1923) to work in con- 
junction with brushes of the epipharynx and hypopharynx and a basket- 
like structure on the hypopharynx “as grinders of the minute pollen 
grains or other fine vegetable matter which forms the food of the imago.” 
The maxillae have a typical generalized structure, each being composed 
of a basis formed of a cardo and stipes (C, Cd, St) and provided with a 
long palpus {Pip) and two terminal lobes {Ga, Lc). The lacinia (Lc), 
however, is much smaller than the galea (Ga). The labium is rudi- 
mentary in that its median part is reduced to a simple lobe, but it bears 
two large three-segmented palpi. 

In the Eriocranidae, a related family, the mouth parts show their 
origin from the micropterygid type of structure, but they take on the 


Fio. 168 . — Head of peach-tree borer moth, Synantkedon exiliosa. 

peculiarities of the typical lepidopterous feeding organs. Mandibles 
are present in the adult stage, though they are very small and probably 
functionless. In the pupa, however, they are extraordinarily large 
(Fig. 167 F, Md) and are so constructed that they open forcibly outward, 
thus enabling the pupa to use its jaws for liberating itself from its tough 
underground cocoon and for digging upward to the surface of the earth. 
The maxilla of the adult (D) has a large six-segmented palpus (Pip) 
and a long slender galea {Ga), but the lacinia is absent. The galeae of 
the two maxillae are grooved on their opposing surfaces, which are 
joined to form a curved proboscis capable of being partly coiled. The 
labium is a simple lobe with long three-segmented palpi (E). 

The Typical Feeding Mechanism of Moths and Butterflies. — The 
feeding mechanism characteristic of the Lepidoptera is a simple device 
for extracting nectar from the depths of flower corollas. It consists 
essentially of a long tube, the proboscis (Fig. 168 A, B, Prb), arising from 
the oral region of the head, where its lumen opens into the mouth (Fig. 
169 F, mth), and of a pumping organ {Pmp) formed of the anterior part 





THE ORGANS OF INGESTION 


309 


of the stomodaeum. Though nectar is the principal food of adult moths 
and butterflies, the feeding apparatus serves as well for imbibing exposed 
liquids, such as water and the juices of fruits. Many species, however, 
take no food of any kind, and in such species the mouth parts are usually 
reduced, in some they are rudimentary and functionless, and the mouth 
pump is entirely absent, the stomodaeum being reduced in the head to a 
threadlike tube. 

The Lahrum . — The labrum is never large; usually it is a narrow trans- 
verse band at the lower edge of the large clypeal region of the face (Fig. 
169 A, C, D, Lm). On its lateral extremities it bears a pair of small, 
hairy lobes, the pilifers (A, P/), which are present likewise in the 
Micropterygidae (Fig. 167 B, P/). 

The Mandibles . — Rudiments of the mandibles occur in some macro- 
lepidopterous moths as small immovable lobes projecting from the 
cranial walls at the sides of the labr um (Fig. 169 D, Md). The reduction 
of the jaws from the larval size takes place at the transformation from 
the caterpillar to the pupa, and again at the change from the pupa to the 
moth. In most species the mandibles are entirely obliterated in the 
adult. 

The Proboscis . — The essential external part of the sucking apparatus 
is the proboscis. This organ is formed of the greatly elongate terminal 
lobes of the maxillae, which, as we have seen, are probably the galeae, 
the laciniae being reduced and suppressed. The proboscis is thus com- 
posed of two lateral pieces, which are held together by interlocking 
grooves and ridges (Fig. 169 E). The opposed walls are thickened and 
concave and enclose between them a canal (fc) through which the liquid 
food or drink of the insect is drawn up to the mouth by the stomodaeal 
pump (F, Pmp). The basal part of each maxilla usually shows a division 
into a small cardo (B, Cd) and a larger stipes {St), the latter bearing a 
rudimentary palpus {Pip) and the elongate galea {Ga). 

When the proboscis is not in use it is tightly coiled beneath the head, 
but it can be completely extended in response to a food stimulus. The 
mechanism of extension and coiling, however, is not well understood. 
The outer wall of each half of the proboscis shows a closely ringed 
structure produced by a succession of sclerotic arcs alternating with 
narrow membranous spaces. This structure probably allows the coiling 
of the tube. Within each half of the latter there is a series of short 
muscle fibers arising near the middle of the outer wall (Fig. 169 G, mcls) 
and extending obliquely distad and toward the inner edge of the concave 
side of the organ, on which they have their insertions. The muscles 
occupy the entire length of each half of the proboscis, and their arrange- 
ment suggests that they serve to coil the proboscis. Unless there is 
some mechanical principle here involved that is not yet understood, we 



310 


PRINCIPLES OF INSECT MORPHOLOGY 


must assume, then, that the proboscis is extended by blood pressure, in 
the same Avay that a toy paper “snake” is unrolled by inflating it, and it 
must be observed that the natural uncoiling of the lepidopterous pro^- 
boscis, beginning at the base and progressing toward the tip, has a strik- 
ing resemblance to the unrolling of the inflated “snake.” The mechanism 
for creating the assumed blood pressure, however, is not evident. 

With species that do not feed in the adult stage the proboscis is usually 
short and weak, and in some forms the entire maxillae are reduced to 
small lobes projecting at the sides of the mouth (Fig. 169 C, Mx). 



Fio. 1C9. — Mouth parts and sucking apparatus of adult Lepidoptera. A, Synaniht,- 
don exitiosa, labrum, epipharynx, and pilifers. B, same, base of maxilla. C, Malacoaoma 
amcricana, shomng rudimentary maxillae. D, Hyphanlria cunea, head and proboscis. 
E, cross section of proboscis of Danais archippus. {From Burgess, 1880.) F, section of 
head of sphinx moth, shoiv-ing sucking pump, diagrammatic. G, diagram of part of 
proboscis and its muscles. 

The Labium . — In all adult Lepidoptera the labium has the simple 
form it has in the Micro pterygidae and Eriocranidae (Fig. 167 E), 
being at most a small lobe or flap, but often it is reduced to a mere mem- 
branous area behind the base of the proboscis supported posteriorly on a 
h 3 ^postomal bar uniting the postgenal areas of the epicranium. The 
three-segmented labial palpi, however, are usually well developed and 
covered with long hairlike scales, forming thus two conspicuous brushes 
projecting upward at the sides of the proboscis. 

The Suckmg Piawp . — A sucking organ is highly developed in adult 
Lepidoptera having functional mouth parts; but with species in which 
the proboscis is rudimentary or absent, the pump is likewise rudimentary, 
the mouth opening into a small funnel leading into the simple slender 
stomodaeum. The functionally developed pump of the Lepidoptera 
includes the buccopharyngeal region of the stomodaeum, as it does in 
Hj'menoptera, since the frontal ganglion lies on its dorsal wall, and the 




THE ORGANS OF INGESTION 


311 


dorsal dilator muscles are inserted before and behind the connectives of 
the ganglion; but it is to be suspected that the anterior part of the organ 
may be formed by the cibarium. The sucking pump of the Lepidoptera, 
however, has been but little studied, and no definite statement can be 
made as to its morphology until we have more comparative information 
on its structure and the relation of its muscles to the head wall. 

In the sphinx moth the sucking pump is a large bulblike structure 
with strongly muscular walls lying in the anterior part of the head (Fig. 
169 F, PiU'p). It opens on the base of the proboscis through a narrow 
neck and tapers posteriorly into the oesophageal tube. The first dilator 
muscle {!) consists of a transverse sheet of fibers arising on the lower 
edge of the clypeal region of the head wall and inserted on the neck of 
the pump. Above these muscles is a median mass of fibers { 2 ) inserted 
on the anterior end of the bulbous part of the pump. The principal 
dilators, however, comprise two thick, paired bundles of fibers (S) arising 
on the upper part of the facial region of the head and inserted laterally 
on the dorsal wall of the pump anterior to the frontal ganglion and its 
connectives. Posterior to the frontal ganglion are a pair of smaller 
muscles (4) and a single median muscle (5), the insertions of which show 
that the rear part of the sucking organ at least is the pharynx, as it is in 
Hymenoptera. 


6. THE FEEDING MECHANISM OP DIPTERA 

The differentiation between larval and imaginal structures in insects 
has reached its highest degree in the mouth parts of the Diptera, for 
here both adults and larvae have widely diverged from the ancestral 
norm, and in the higher families the maggots have outdone the flies. 
In fact, in the Cyclorrhapha it appears that the larvae have developed 
not only a feeding mechanism but also a functional head and mouth that 
have little relation to the cephalic structures of the imago. The true 
mouth parts of the fly are entirely suppressed during the whole larval 
period, the major part of the head is invaginated into the body, and a new 
set of organs is developed to serve the purposes of the maggot. The 
imaginal parts are restored during the pupal transformation and then 
developed directly into the specialized form characteristic of the adult. 
In the more generalized Orthorrhapha, however, the larva retains the 
usual head structure and a feeding mechanism that clearly demon- 
strates the origin of the Diptera from insects having typical biting mouth 
parts. 

Mouth Parts of Dipterous Larvae 

Only the extreme types of dipterous larval mouth parts will be 
described here, one representing the most generalized form, occurring 



312 


PRINCIPLES OF INSECT MORPHOLOGY 


in the Orthorrhapha, the other the highly specialized structure developed 
in the muscoid maggot. The intermediate stages between these extremes 
are still not well understood and offer an inviting field for further 
exploration. 

The Orthorrhaphous Type of Larval Mouth Parts. — In the larvae of 
Tipulidae or Tabanidae and related families the head is an elongate 
oval capsule with strong sclerotic walls, but it is almost completely 
retracted into the anterior part of the thorax (Fig. 170 A), where it is 
enclosed in a membranous sheath formed by an inflection of the neck 
membrane {Cv). The dorsal wall of the head, however, is much longer 
than the lateroventral walls, which taper anteriorly and are united below 
in a small, toothed, triangular hypostomal lobe {Hst) projecting beneath 
the mouth {Mih) and the rudimentary labium {Lh). The brain and the 
suboesophageal ganglion are withdrawn from the head and lie in the 
middle of the thoracic region of the body, but long nerve trunks extend 
forward from them to the organs of the head normally innervated by 
these ganglia. 

The mandibles in the tipulid larva are strongly musculated jaws of 
the generalized biting type of structure (Fig. 170 B). In the tabanid 
larva the mandibles are rudimentary, but in various other families of 
the more generalized flies, as in the Chironomidae and Culicidae, the 
larval mandibles are also jawlike in form. 

The maxillae of the tipulid larva are small flat lobes (Fig. 170 C), 
in which the usual parts of a maxilla are somewhat indefinitely separated. 
In the larva of Tabanus, however, the maxillae have a more generalized 
structure, each organ (D) comprising a basal lobe (Cd), which is appar- 
ently the cardo, and a larger stipes {St) bearing a distinct galea {Ga) and 
a lacinia (Lc), but the palpus is absent. 

The labium is rudimentary in all fly larvae, and the hypostomal 
lobe of the ventral head wall is frequently mistaken for it. In the larva 
of Ti'pula the labium is a small median projection beneath the mouth 
(Fig. 170 A, Lh), but it is concealed above the hypostoma {Hst). United 
with the labium is a small hypopharyngeal lobe {Hphy), and between the 
latter and the labium opens the duct of the salivary glands {SID). 

The head stomodaeum of the tipulid larva is a straight tube (Fig. 170 
A, Stom), slightly widened anteriorly, but showing no structural differ- 
entiation into buccal, pharjmgeal, and oesophageal regions. On its 
dorsal wall are inserted three groups of dilator muscles. The fibers of 
the first group {dlcb, dlbc) belong to the cibariobuccal region ; those of the 
second and third groups {dlphy) correspond to the dilators of the anterior 
pharyngeal region in other insects (Fig. 155, Phy), since they are inserted 
posterior to the connectives of the frontal ganglion {FrGny) ard are 
precerebral in position, the brain being withdrawn into the thorax. 



THE ORGANS OF INGESTION 


313 


The Muscoid Type of Larval Mouth Parts. — In the higher cyclor- 
rhaphous Diptera the usual mouth parts are entirely suppressed in the 
larval stage, and the only external feeding organs of the maggot are 
a pair of strong mouth hooks movable in a vertical plane. Moreover, 
the entire facial region of the head posterior to the clypeus, including 
the area of the Irons and that of the imaginal antennae and compound 
eyes, is invaginated (not merely retracted) into the thorax, and a circular 
fold of the neck projects beyond the mouth to form a conical snout, which 
is the functional “head” of the maggot. 



Fig. 170. — Head and mouth parts of larval Diptera. A, Tipula abdominalis, dia- 
grammatic section of retracted head. B, same, right mandible, mesal rdew. C, same, 
left maxilla, outer view. D, Tahanus punclifer, left maxilla, outer ■view. E, F, G, dia- 
grams illustrating stages in the invagination of the head of a muscoid maggot. 

To understand the morphology of the extraordinary head structure 
of the cyclorrhaphous maggot, we must trace its evolution from the head 
structure of a simpler type, possibly from an invaginated head of the 
tipulid or tabanid variety (Fig. 170 E). First, we must assume that the 
neck membrane (Cv), between the points x and y above, and x' and y' 
below, has been extended in a fold (F, Cv) somewhat beyond the mouth 
{mth) of the retracted head, while, at the same time, the areas of the 
dorsal wall of the head on which the antennae and compound eyes of 
the adult are to be formed have become invaginated posterior to the 
clypeus (F, Clp) as a pair of lateral pouches (aop, only the right pouch 
shown in the figure), which contain the histoblastic rudiments of the 
antennae and compound eyes. Next, apparently, the cleft between 



314 


PRINCIPLES OF INSECT MORPHOLOGY 


the two antenno-ocular pouches has been extended forward through the 
cl 5 TDeal region, di^nding the latter mesally into two lateral plates covered 
by the closely appressed neck membrane, forming thus two flat, double- 
walled wings on which the cl 5 rpeal muscles {dlch) take their origin, and 
from the posterior ends of which the antenno-ocular pouches (oop) extend 
into the thorax. Finally, by a further extension of the neck fold, there 
is established within the latter a preoral cavity, known as the atrium 
(G, Air). The functional mouth {mth) is thus situated at the inner end 
of the atrium, and the closed passages from the antenno-ocular pouches 
over the wings of the clypeus are confluent at the labrum (Lm) in a dorsal 
diverticulum of the atrium just before the mouth. The neck fold 
enclosing the atrium now becomes the apparent larval head. Below 
the oral aperture is a pair of very small ectodermal pouches said to 
contain the histoblasts of the imaginal labium {Lh), the labium being 
suppressed as an external organ in the larva in the same way as are the 
thoracic appendages. Just within the lips of the atrial opening there 
project, one on each side, the two mouth hooks {mhk), the substitute 
jaws of the maggot. 

Along with the numerous transpositions that so alter the cephalic 
structure of the cyclorrhaphous larva, the ingestive apparatus becomes 
highly developed and specialized. In the tipulid larva (Fig. 170 A) it 
is to be observed that the cibarium (Cb) has a typically orthopteroid 
position and structure, and that the cibarial and buccal dilators {dlcb, 
dlbc) form a distinct group of muscles separated from the true pharyngeal 
dilators (dlphy) by the frontal ganglion connectives. In the cyclor- 
rhaphous larva (G) the cibarium is transformed into a large sucking 
pump {Pmp), with its great mass of dilator muscles (dlcb) arising on the 
invaginated clypeal surface of the head (Clp). The oriflee of the pump 
(mih) is the functional mouth, but the true mouth is the opening from 
the pump lumen into the stomodaeum (Stom). The floor of the pump 
is the base of the hypopharynx, and the salivary duct (SID) opens in the 
normal position between the hypopharynx and the labial rudiment (Lb). 
The lateral walls of the pump and the walls of the clypeal wings leading 
back to the antenno-ocular pouches (aop) become strongly sclerotized, 
forming the conspicuous sclerotic structure lying in the anterior end of 
the maggot, commonly known as the “pharyngeal skeleton” or “bucco- 
pharyngeal armature.” 

The mouth hooks of the cyclorrhaphous larva (Fig. 170 G, mhk) are 
often called “mandibles,” but since they are solid cuticular structures, 
shed with each moult, arising from the lips of the atrial cavity (Air), 
which is evidently derived from the infolded neck membrane, it is not 
clear how the larval jaws can have any relation whatever to true man- 
dibles. Furthermore they lie in vertical planes and are moved by muscles 



THE ORGANS OF INGESTION 


315 


taking their origins on the lateral walls of the cibarial pump. The mouth 
hooks thus appear to be secondary cuticular structures developed for the 
purposes of the maggot. After the third moult they are not renewed. 

The accompanying diagrams (Fig. 170 E, F, G) perhaps do not express 
accurately all the relations in the larval head structures, since there are 
still certain points that are obscure, but the known facts of development 
and of comparative anatomy demonstrate that in some such way as that 
indicated the peculiar structures of the cyclorrhaphous maggot have been 
evolved from the more usual type of head structure. Essentially, the 
condition is one in which not only the appendages but a large part of the 
head as well have been reduced to histoblastic rudiments and withdrawn 
into the body during the larval stage, to be everted later during 
the pupal transformation in order to complete their dcA’^elopment into the 
imaginal parts. In most insects the invaginations that contain 
the imaginal histoblasts do not appear until the last larval stage; in the 
higher Diptera they are formed in the embryo and thus become a part 
of the larval structure. A much more primitive condition occurs in some 
of the lower Diptera, as in Chironomus and Psychoda, in which the 
antenno-ocular pouches are formed only in the pupal integument devel- 
oped in the last larval instar, while the other features of the cycior- 
rhaphous larva do not appear at all. 

The Feeding Mechanism of Adult Diptera 

No adult dipteron has mouth parts of the typical biting type of 
structure. Though certain flies are said to “bite,” the effect is the 
result of a puncture and not of a pinch. The majority of flies are inca- 
pable of inflicting any kind of wound. The familiar “biting flies” 
belong to two groups ; in one group the mandibles are the piercing organs, 
in the other the labium is the effective instrument. Mandibles occur 
only in a few of the more generalized families of Diptera, being present 
in the females of Phlebotomus (Psychodidae), Dixidae, CuUcoides (Chi- 
ronomidae), Culicidae, various species of Simuliidae, and in Tabanidae. 
Among male flies mandibles are said to occur only in certain species of 
Simulium. Flies having a piercing labium include principally the tsetse 
fly, stable fly, and horn fly of the family Muscidae. The robber flies 
(Asilidae) also should be included among the biting flies, though they 
confine their attacks to other insects. The piercing organ of the robber 
flies, however, appears to be the hypopharynx, which is long, sharp 
pointed, and protractile. The only truly biting flies are certain species 
of Dolichopodidae, in which the terminal lobes of the labium are strongly 
sclerotized and jawlike in form and action. 

The best known of the mandibulate piercing flies are the female horse 
flies (Tabanidae), and female mosquitoes. The mouth parts of a horse fly 



316 


PRINCIPLE^; OF INSECT MORPHOLOGY 


may be studied as an example of the more generalized condition of the 
mouth parts as it occurs in adult Diptera. To understand the entire 
feeding mechanism, ho^Yever, it will be necessary to know also something 
of the structure of the clypeal region of the head. 

The Feeding Organs of a Horse Fly. — The mouth parts of the horse fly 
form a compact group of organs projecting downward from the peristomal 
margin of the head (Fig. 171 A). In the female fly there are nine pieces 
included in the group, three of which are median and unpaired, while 
the other six represent three pairs of lateral organs. The most anterior 
of the median organs is the broad, dagger-shaped labrum (Lm). Its 
lateral edges are overlapped in the usual position by a pair of large two- 
segmented palpi (MxPlp), which belong to the maxillae. Posterior to 
the labrum are two pairs of long, slender, tapering blades, the anterior 
of which are the mandibles (Md), the posterior the lobes of the maxillae 
{Mx). Posterior to the mandibles and between the maxillae is a second 



Fio. 171. — Head and mouth parts of an adult horse fly, Tabanus airatus. A, anterior 
view. B, details of clypeus and labrum, C, labium and maxillae, posterior view. 

median piece resembling the labrum but slenderer. This is the hypo- 
pharynx. Behind and partly enclosing all these parts in the normal 
position is the large median labium (Lb) ending in two broad lobes. 

The Labrum and the Clypeus . — The labrum of the horse fly, as noted 
above, is shaped like the blade of a dagger, but it is not particularly 
rigid, and its point is blunt (Figs. 171 A, B, 172 A, Lm). Proximally 
it is attached to the lower margin of the facial area of the head by a 
median membranous area and two divergent lateral arms. A short 
median apodeme projects dorsally into the head cavity from the anterior 
wall of the labrum and gives insertion to a fan-shaped muscle arising on 
the clj’peal plate of the head (Fig, 171 B, clp). This muscle serves 
apparentl}’' to keep the labrum in close contact with the other pieces of 
the mouth parts. The presence of a clypeolabral muscle is a special 
feature of the Diptera and constitutes an exception to the general rule 
that the labral muscles take their origin on the frons. The posterior, or 
epipharyngeal, wall of the labrum is excavated by a median channel 



THE ORGANS OF INGESTION 


317 


jontinued proximally into the small mouth aperture located behind the 
base of the labrum (Fig. 172 A, mth). 

When the mouth parts of the female horse fly are in the normal 
position, the labral groove is closed posteriorly by the broad, overlapping 
mandibles, and there is thus formed a tubular passage leading up to the 
mouth. This conduit is the food meatus, which, in female Tabanidae, 
as pointed out by Vogel (1921), lies thus between the labrum and the 
mandibles. In the female mosquito, however, Vogel shows, the labrum 
itself forms the food canal since its concavity is closed posteriorly by the 
approximation or overlapping of its incurved lateral margins. 

In many of the Dolichopodidae the epipharyngeal wall of the labriun 
bears an armature of spines or movable teeth, the latter being highly 
developed in the genus Melanderia; but since, in general, the posterior 
surface of the dipterous labrum is smooth and presents no structure of 
any kind to be specifically termed an epipharynx, the writer sees no 
reason for following the usual custom of calling the elongate labral lobe 
of the Diptera a “labrum-epipharynx.” 

The area of the head wall in Tabanus from which the labrum is sus- 
pended is a median part of the cl 3 q)eus (Fig. 171 B, clp) separated from 
the lateral parts of the cl3q5eal area by a membranous fold on each side. 
The lateral limits of the true clypeus (Clp) are marked by the long slitlike 
anterior tentorial pits (at, at), from which the epistomal suture (es) is 
arched upward and crosses the lower part of the face beneath the bases 
of the antennae. Upon the median cl 3 peal plate (clp), as we shall 
presently see, the dilator muscles of the sucking pump take their origin. 
In the Tipulidae the clypeal area forms the upper wall of a snoutlike 
projection of the head extending anterior to the eyes, and in some species 
bearing a strong spine-like process near its distal end. In the higher 
Diptera the median part of the clypeus becomes an independent sclerite, 
but the dilator muscles of the pump retam their attachments upon it. 

The Mandibles . — The mandibles of the horse fly are long, flattened, 
sharp-pointed blades, their tips reaching to the apex of the labrum 
(Fig. 171 A, Md). Each is articulated by an expanded base (Fig. 172 B) 
to the lower edge of the head at the sides of the mouth and is pro^dded 
with antagonistic muscles that take their origin on the head wall. The 
mandibles of the fly are thus, as in biting insects, capable of being moved 
in a transverse plane, but they have no movements of protraction or 
retraction. The thrust of the piercing mandibles is made by a forceful 
action of the head and body of the fly. 

The Maxillae . — ^Each maxilla consists of a basal part evidently com- 
posed of the cardo and stipes (Fig. 172, C, Cd, St), of a large, thick, 
two-segmented palpus (Pip), and a long, slender, tapering blade (Ga). 
The maxillary bases underlap the proximal part of the labium (Fig. 171 C, 



318 


PRINCIPLES OF INSECT MORPHOLOGY 


Pmt) and are articulated by their cardinal extremities to the lower edges 
of the postgenae (a") just below the posterior tentorial pits {pt, pt). 
The palpus {MxPlp) arises by a narrow stalk from the outer edge of the 
stipes. Beyond the base of the palpus the stipes is continued into the 
long blade-like maxillary lobe. The latter (Fig. 172 C, Go) may be 
regarded as the galea, sinee it appears to correspond to the principal 
lobe of the maxilla in Hymenoptera and Lepidoptera. Near its base 
it gives off a small strip (a) that connects mesally with the head wall. 
Patten and Evans (1929) regard this small lobe in Haematopota as the 
lacinia, but its origin from the base of the larger lobe makes this interpre- 
tation seem doubtful. The maxiUae are well developed in many orthor- 
rhaphous flies that lack mandibles. 



Fig. 172. — Mouth parts of Tabanus alratus, separated, lateral view. A, labrura 
and sucking pump (cibarium) with dilator muscles arising on clypeus. B, left mandible. 
C, left maxilla. D, labium, hypophariuix, and salivary syringe. 

The Hypopharynx and the Salivary Syringe. — The hypopharynx is a 
long, narrow, tapering stylet (Fig. 172 D, Hphy) arising from the ventral 
wall of the head just behind the mouth aperture. Normally it lies in the 
deep groove of the anterior face of the labium. It is traversed by the 
outlet duct of the salivary glands (sm), which opens by an aperture on its 
tip (Slo). Just proximal to the base of the hypopharjmx the duct is 
enlarged to form a syringe-like apparatus (Syr), the anterior wall of 
which is inflected into the lumen and is provided with dilator muscles 
(dlsyr) arising on the posterior surface of the sucking pump of the feeding 
apparatus (A, Prnp). Since the food pump of Diptera represents the 
cibarium of orthopteroid insects (Fig. 155, C6), its floor is formed by the 
adoral surface of the hjT^opharynx. The dilator muscles of the salivary 
sjTingc, therefore, are the usual hypopharyngeal muscles (Is) of the 
salivarium; the ventral, or labial, muscles are absent. Just how 
the syringe and the salivary channel have become enclosed within the 
hj-popharynx cannot be ex-plained. The salivary glands of the Tabanidae 



THE ORGANS OF INGESTION 


319 


are long, simple, tubular organs; their secretion is said to contain a 
powerful blood anticoagulin (see Patten and Evans, 1929). 

The Labium. — The labium of the horse fly is a large, thick, elongate 
appendage (Eig. 172 D, Lb) ending in two broad lobes known as the 
lobelia (La). It is suspended from the posterior part of the head by a 
membranous basal region (Fig. 171 C, Pmt) lying between the maxillary 
stipites and is separated from the foramen magnum (For) by a narrow 
hypostomal bridge (Hst) connecting the posterior angles of the postgenae. 
The stalk of the free part of the organ is the stipital region of the append- 
age and is therefore the prementum (Figs. 171 C, 172 D, Prmt). It is 
deeply evacated longitudinally on its anterior surface. In the normal 
position of the mouth parts, the labrum, the mandibles, the hypopharynx, 
and the maxillae all lie within the cavity of the labium, the hypopharynx 
immediately behind the mandibles, with the maxillae to either side of it 
but posterior to its broad lateral margins. The relations of the several 
elements of the mouth parts to one another are best seen in a cross section 
(see Vogel, 1921). 

The terminal lobes of the labium, or labella, are large soft pads capable 
of being spread outward from the end of the stipital stalk (Fig. 172 D) 
to form a broad disc, sometimes called the “oral sucker.” The posterior 
halves of the labella are united, but their anterior parts are separated 
by a deep median cleft. The under surface of each lobe is marked by the 
lines of numerous close-set, transverse channels in its membranous wall, 
called 'pseudotracheae from their superficial resemblance to half-open 
tracheal tubes. The mesal ends of the canals lead to the base of the cleft 
between the labellar lobes, at which point normally lies the apex of the 
labrum. In the feeding fly, the blood collected by the labellar channels 
is here taken into the food canal between the labrum and the mandibles. 

In the genus Melanderia of the Dolichopodidae the labella have a 
very unusual development (see Aldrich, 1922; Snodgrass, 1922). Each 
labellum has a strongly sclerotized movable lobe with the free, sharp 
apical part turned inward. The two lobes give the appearance of a pair 
of mandibles, and they are provided with muscles arising in the premen- 
tum so attached on the lobes that the latter apparently can be opened 
and closed in the maimer of a pair of jaws. Since the Dolichopodidae 
are predacious, it is highly probable that the labellar lobes of Melanderia 
serve to grasp and hold the living prey on which the insects feed. 

The labella of the fly labium have been generally regarded as the 
paraglossae, apparent rudiments of the glossae being sometimes present 
between them; but Crampton (1923, 19255), from a comparative study 
of the labium in Diptera and related insects, has given reasons for believ- 
ing that the labellar lobes are the labial palpi. Palpi, however, are 
typically provided with antagonistic muscles; the lobes of the fly labium 



320 


PRINCIPLES OF INSECT MORPHOLOGY 


have usually each only one muscle inserted directly upon it. The term 
“labella” is used by some -writers in a singular sense, but it is properly 
the plural of labellum (diminutive of ‘Tip”), which is the Latin form of 
the singular. 

The Sucking Pump. — The sucking apparatus of the horse fly is a small 
chamber -which extends upward in the lower part of the head from the 
functional mouth (Fig. 172 A, Pmp). The posterior and wide lateral 
walls of the organ are strongly sclerotized and fixed to the upper end of 
the labrum. The anterior wall, on the other hand, is thin and flexible 
and is ordinarily deeply invaginated into the lumen of the pump, but it is 
provided with two large groups of muscle fibers taking their origin on the 
median clypeal plate of the head wall (Fig. 171 B, dp) and is thus capable 
of exerting a sucking action on the liquid food ascending to the mouth 
through the food canal of the mouth parts. The origin of the dilator 
muscle of the pump on the median plate of the clypeus shows that the 
sucking apparatus of the Diptera is the cibarium of orthopteroid insects, 
together with its dilator muscles and the clypeal plate on which these 
muscles take their origin. The functional mouth aperture (mth) leading 
into the pump chamber, therefore, is not the true mouth, the latter being 
the opening into the stomodaeum at the inner end of the pump. 

The Muscoid Types of Mouth Parts. — In the higher Diptera the 
external feeding apparatus of the adult fly consists of a proboscis. The 
proboscis is a composite structure formed of the labrum, the h 3 q) 0 - 
pharynx, and the labium, all supported on a membranous base that 
contains in its anterior wall the median clypeal plate on which arise 
the dilator muscles of the sucking pump. The proboscis supports a 
pair of palpi, which are probably the maxillary palpi, and it terminates 
in a pair of labellar lobes. Two functional types of structure are dis- 
tinguishable in the proboscis, depending on the nature of the labellar 
lobes. In most of the muscoid flies the labella are broad, soft pads 
resembling those of the horse fly, and such species are incapable of biting, 
though some are provided with small labellar teeth that enable them to 
rasp the food substances. Flies ha-ving mouth parts of this kind are 
designated by Metcalf and Flint (1928) as the sponging type of Diptera. 
With these flies the proboscis is usually flexible and extensible and when 
not in use is folded against the lower part of the head or retracted into a 
ventral ca\’ity of the head wall surrounded by the projecting margins 
of the peristome. In a few species of the family Muscidae the proboscis 
is rigid, and the labellar lobes are small, cutting plates. These species 
constitute the so-called “biting” muscoid flies. 

The Nonpiercing, or Sponging, Type of Muscoid Mouth Parts. — The 
typical muscoid proboscis, as seen in the blow fly or house fly (Fig. 173 A, 
Pfb), consists of three parts: first, a large basiproboscis, or rostrum (Bst)] 



THE ORGANS OF INGESTION 


321 


secoad, a mediprohoscis, or haustellum (Hstl) ; and, third, a disiiproboscis, 
formed of the labella (La), or lobes of the so-called "oral sucker.” 

The rostrum is a broad inverted cone having for the most part flexible 
membranous walls. On the upper part of its anterior surface, however, 
there are usually two median plates (Fig. 174, C, c, dp). The more 
ventral of these plates {dp), having the form of an inverted V, is a con- 
stant feature of the rostrum in the muscoid proboscis; the upper plate (c) 
is a weaker sclerotization hinging the V-shaped plate to the lower edge 
of the cranial capsule. On the lower part of the anterior face of the 
rostrum are two small lateral sclerites (mxpl), which support the pair of 
long palpi (MxPlp). 

The cylindrical haustellum projects downward and somewhat forward 
from the end of the rostrum when the proboscis is protracted; in the 



Fig. 173. — Head and proboscis of the house fly, Musca domesiica. A, lateral view 
of head with proboscis extended. B, anterodistai view of extended proboscis, showing 
ventral surfaces of labella -with aperture (o) leading into food meatus [fm) between labrum 
and hypopharynx. 


retracted condition it folds upon the anterior surface of the rostrum. 
The posterior wall of the haustellum is occupied by a prominent plate 
known to students of Diptera as the thyroid or "mentum.” The anterior 
surface is covered by a long, tapering, strongly sclerotized flap (Figs. 
173 A, 174 C, Lm), which arises from the distal margin of the rostrum and 
is partly overlapped by lateral folds of the haustellum. This flap is the 
labrum. By lifting its distal end (Fig. 173 B, Lm) there is exposed in the 
anterior part of the haustellum a deep, lengthwise cavity in which lies 
the blade-like hypopharynx {Hphy). Between the labrum and the 
h3q?opharynx is the food canal (fm) of the proboscis, which leads up to the 
functional mouth situated behind the base of the labrum as in the horse fly 
(Fig. 172 A, mth). 

The labellar lobes (Fig. 173, La) terminating the proboscis in the 
nonpiercing, or sponging, type of muscoid mouth parts are broad pads 



322 


PRINCIPLES OF INSECT MORPHOLOGY 


similar to those of Tdbanus. When spread out flat they form an oval 
disc (B, La) crossed by the pseudotracheal channels (&), and enclosing 
centrally an opening (a) at the posterior end of the anterior cleft between 
the component lobes. This opening, which leads into the food canal of 
the proboscis, is known as the “oral aperture,” but it must not be con- 
fused with the functional mouth aperture of the fly, which, as we have 
seen, lies at the upper end of the food canal between the bases of the 
labrum and hypophar 3 mx. The cleft between the labellar lobes anterior 
to the aperture of the food canal is known as the prestomum. Its inner 
walls in some flies are armed posteriorly with several rows of small 
prestomal teeth. 

The morphology of the muscoid proboscis is difficult to understand 
in all its details, and there are many features in the structure and muscu- 
lature of this complex feeding apparatus which show that it is a highly 
specialized composite organ. The parts of the proboscis distal to the 
rostrum are clearly the homologues of corresponding elements in the 
mouth parts of the horse fly, including the premental region and terminal 
lobes of the labium, together with the labrum and the hypopharynx. 
The composition of the rostrum, however, is less easy to determine, but 
it apparently includes the base of the labium and a part of the clypeal 
region of the head. 

The inverted V-shaped plate of the anterior wall of the rostrum 
(Fig. 174, C, D, clp) bears upon its lateral arms the origins of the dilator 
muscles of the cibarial pump (D, 3). There can be little question, 
therefore, that this sclerite represents at least the median part of the 
cl 3 peus in the head of Tabanus (Fig. 171 B, clp). The lateral walls of 
the pump in the muscoid flies, however, are attached by wide apodemal 
inflections to the margins of the V-shaped clypeal plate, and for this 
reason anatomists have often regarded the latter as a part of the sucking 
mechanism. The pump, the clypeal plate, and the connecting apodemes 
are described as a stirrup-shaped structure which, in the special termin- 
ology of the fly head, is called the fulcrum, because certain muscles 
attached upon it serve to flex the proboscis (see Lowne, 1890-1895; 
Graham-Smith, 1930). Frew (1923) recognizes "this exposed part of 
the fulcrum” as the cljqjeus. Peterson (1916), on the other hand, 
attempted to explain the anterior plate of the rostrum as derived from 
the tormac, which are sclerotic processes in the base of the epipharyngeal 
wall of the labrum. The attachment of the dilator muscles of the 
cibarial pump on the arms of the V-shaped rostral plate, however, clearly 
demonstrates the clj'peal origin of this sclerite, and confirmatory e^ddence 
of its homologj' vith the median clj'^peal region in Tabanus is seen in the 
fact that a pair of labral muscles (Fig. 174 D, 2) take their origin on its 
dorsal part. The smaller sclerite above the V-shaped clypeal plate 



THE ORGANS OF INGESTION 


323 


(C, c) is either a part of the cl 3 rpeus or a secondary sclerotization hinging 
the latter to the lower margin of the face. We must, then, assume that 
the rostrum of the fly’s proboscis includes at least a part of the clypeus, 
which has become detached from the head walls and shifted ventrally. 

The origin of the maxillary palpi (Fig. 174 C, MzPlp) from small 
sclerites in the ventral part of the rostrum shows that the last has also 
absorbed the basal parts of the maxillae. The remaining major part of 
the rostrum is apparently to be attributed to the basal part of the 
labium. 



Fia. 174 . — Proboscis of a fruit By and a blow By. A, Rhagoletis pomonella, proboscis, 
anterior view. B, same, head and proboscis, lateral view. C, CalKphora erythrocephala, 
proboscis, anterior view. D, same, head and proboscis, lateral view, showing muscles. 
{Adapted from Graham-Smith, 1930.) 

Within the rostrum lie the sucking pump, the salivary duct with its 
syringe, and a pair of long rodlike apodemes arising from the basal angles 
of the labrum (Fig. 174 C, D, Ap). The sucking pump of the blow fly 
does not differ from that of the horse fly, except for the connection of its 
lateral walls with the margins of the cliTJeus and in details of its form. 
The rostral apodemes appear to belong to the labrum, but some writers 
have regarded them as remnants of the maxillae. Muscles are inserted 
upon them wliich have no apparent homologues in the head of biting 
insects. 

The musculature of the proboscis is somewhat complex (Fig. 174 D) 
but is not difficult to study. Most of the muscles, however, appear to 
be special adaptations to the functions of the fly proboscis, and the.y 
cannot be satisfactorily homologized vith the muscles of the moutlx 
parts in biting insects. 



324 


PRINCIPLES OF INSECT MORPHOLOGY 


For a full account of the structural details and musculature of the 
head and mouth parts of muscoid flies the reader is referred to the 
elaborate paper on the proboscis of the blow fly by G. S. Graham-Smith 
(1930), and to that on the head and mouth parts of the tsetse fly by 
Jobling (1933), though the many special terms used by these writers, 
taken largely from Lowne (1890-1895), will be somewhat confusing to 
the student of general insect morphology. 

The blow fly Calliphora erytlirocephala, according to the observations 
of Graham-Smith, has several different methods of feeding, which involve 
a use of the labellar lobes in as many corresponding different positions. 
In the nonfeeding position, the lobes are flexed posteriorly against the 
haustellum with their pseudotracheal surfaces in apposition. When 
the fly feeds on a film of liquid the labella are spread out flat like the 
leaves of a book, but only the parts of their surfaces covered by the 
pseudotracheae are applied to the food substance; the liquid is then 
sucked up through the pseudotracheal openings (“interbifid grooves”), 
and “all particles too large to pass through them are filtered out and 
rejected.” This way of using the labella is termed by Graham-Smith 
the filtering position. In some cases the edges of the lobes are turned 
down, producing a marginal rim around the labellar disc, thus giving a 
second or cupping position. By a separation of the labella the prestomal 
teeth may be partly exposed and used to some extent while the liquid food 
is still being filtered through the pseudotracheae. This gives an inter- 
mediate position leading to the next, or scraping position, in which the 
labellar lobes are turned upward until the prestomal teeth are fully 
exposed for the purpose of rasping. In this position the pseudotracheal 
surfaces are out of action. Finally, there is the direct feeding position, 
produced by folding the labella upward and outward against the sides of 
the haustellum, in order that the aperture of the food canal may be 
applied directly to the food, thus allowing not only the liquid but particles 
in the liquid to be freely ingested. 

In some of the nonpiercing Muscidae the prestomal teeth are large and 
strong, as in Musca crassirostris, which attacks cattle and obtains blood 
from them by scratching the skin with its powerful labellar armature. 

The Piercing Type of Muscoid Mouth Parts. — Muscoid flies having the 
biting” type of mouth parts are principally the stable flies, the horn flies, 
and the tsetse flies, that is, members of the genera Stomoxys, Haeniatobia, 
and Glossina. The piercing organ of these flies is the proboscis (Fig. 175 
A, Prb), which consists of the same parts as does that of the nonpiercing 
muscoids. The haustellum, however, is elongate and rigid, swollen at 
the base to accommodate the contained muscles, and tapering toward the 
extremity. The labellar lobes (C, La) are small, flat, and densely horny, 
and the prestomal teeth are well developed. The labrum is almost 



THE ORGANS OF INGESTION 


325 


circular in transverse section (B, Lm) and is firmly locked within the 
upturned edges of the labium {Lh), the two parts forming thus a tubular 
food channel (/m), within which lies the slender hypopharynx (Hphy) 
traversed by the salivary channel (sm). The beaklike proboscis is 
forced into the flesh of the victim by a strong thrust of the head and body 
of the fly, and the blood is sucked up directly through the food canal. 
The salivary secretion of Glossina is said to prevent clotting of the blood. 

For further information on the structure of the mouth parts and the 
feeding mechanism of the piercing muscoid flies, the student should con- 



Fig. 175. — Head and mouth parts of a tsetse fly. A, Glossina palpalis, male, head 
and proboscis. B, G. fusca, cross section of proboscis. {From Vogel, 1920.) C, G. palpalis, 
mouth parts and sucking pump. 

suit the works of Cragg (1912), Hansen (1903), Jobling (1933), Minchin 
(1905), Patten and Evans (1929), Stuhlmann (1907), and Vogel (1920). 

7. THE MOUTH PARTS OF SJPHONAPTERA 

The mouth parts of the fleas appear to be of the dipterous type of 
structure, but they are more generalized than those of any adult fly. 
None of the usual pieces is lacking, and both the maxillae and the labium 
retain long, segmented palpi. The mandibles are said to be the cutting 
and piercing organs of the fleas, and the preoral food canal lies between 
them and the concave under surface of the labrum, as it does in the 
Tabanidae. The essential characters of the flea mouth parts, as described 
by Patten and Evans (1929), are as follows: The labrum is long and 
slender but is blunt at the apex; its lateral edges are rolled downward 
between the mandibles, forming the anterior wall of the food canal. 
The mandibles are long, sharp-pointed blades armed distally with minute 



326 


PRINCIPLES OF INSECT MORPHOLOGY 


teeth. The maxillae are short, rather wide plates, bearing each a long, 
segmented palpus. The labium consists of a short median body, hollowed 
anteriorly, bearing distally a pair of segmented palpi. The short hypo- 
pharynx projects into the proximal end of the food canal between the 
bases of the mandibles; upon it opens the duct of the salivary glands, the 
secretion of which is conveyed to the wound through a channel between 
the posterior edges of the mandibles. 

8. THE FEEDING MECHANISM OF THYSANOPTERA 

There are few insects so isolated from other orders by some peculiar 
feature of their anatomy as are the Thysanoptera and the Hemiptera 
in the form and structure of their mouth parts. It has often seemed to 
entomologists that the Hemiptera in particular must be a group but 
distantly related to other insects, and yet such an assumption is dis- 



femoralis, section of head and beak, showing food meatus (Jm) between labrum and hypo- 
pharj'nx, and sucking pump (Pmp) with its dilator muscles. B. same, mandible. C, 
FTanMiniella iritici, maxilla. D, Heliothrips haemorrhoidalis , maxilla and muscles. 
(A, B, C from Peterson, 1915; T) from Reyne, 1927.) 

credited in all parts of their organization except the mouth parts. It is 
true, however, that the members of these two sucking orders possess a 
piercing mechanism, at least, that has no counterpart in any other group 
of insects, though some of its features are suggested in certain structures 
found in the Psocidae and Mallophaga. 

The mouth parts of the thrips, while aberrant in some respects, are 
distinctly more generalized than are those of the Hemiptera, and studies 
of their development give us an insight into the nature of the unusual 
modifications that have produced the distinctive characters of both the 
Thysanoptera and the Hemiptera. The most important papers on the 
mouth parts of the thrips are those of Peterson (1915) and Reyne (1927). 

The curiously distorted head of the Thysanoptera, which usually is 
produced forward from the thorax with the facial area turned ventrally 
(Fig. 176 A), bears a short, thick, conical beak projecting downward 
from the posterior part of the under surface. Externally the beak is 
'ormed by the labrum in front (Lm), the maxillae on the sides, and the 



THE ORGANS OF INGESTION 


327' 


labium behind (Lb). Within the beak is contained a single mandible, 
which is the left one, two piercing stylets associated with the bases of the 
maxillae, and the h3T)opharynx (Hphy). All the elements of the piercing 
mechanism and the lower parts of the head wall are subject to an asym- 
metry of shape, and they may differ in details of form in the two thy- 
sanopterous suborders, the Terebrantia and Tubulifera, but their 
essential structure is the same in both groups. 

The Labrum. — The labrum (Fig. 176 A, Lm) is a broad, triangular 
lobe, usually of irregular form, covering the anterior surface of the beak. 
Between its wide base and the lower edge of the facial region of the head 
is an asymmetrical triangular sclerite which is probably a basal part of 
the labrum, since, for reasons to be given presently, the vniter would 
regard the clypeus as being contained in the large facial region of the head 
capsule (Clp). 

The Mandible. — The mandible developed on the left side of the head 
is contained in a mandibular pouch invaginated within the head from the 
basal angle between the contiguous surfaces of the labrum and the left 
maxilla. The mandible is an elongate piercing organ (Fig. 176 B), 
consisting of a wider basal part (b) and a slender distal stylet (Stl). On 
the base are inserted retractor muscles arising on the head wall, but 
protractor muscles are said to be absent. A functionless rudiment of the 
right mandible, according to Peterson (1915), is present on the right side 
of the head. 

The Maxillae. — The maxillae of the thrips, with their associated 
stylets, are of particular interest because of the light they have thrown 
on the obscure morphology of the maxillae in the Hemiptera. Each 
maxilla consists of an elongate, triangular plate (Fig. 176 C, St) forming 
the lateral wall of the conical beak. Near its distal end it bears a short 
palpus (Pip) of two or more segments. From the inner side of the base 
of the maxillary plate there is given off mesally (in the Terrebrantia 
at least) a short bar (Ivr), which supports a long, slender maxillary stylet 
(Stl) with an enlarged base, which is contained in a pouch of the ventral 
head wall. It is shown by Reyne (1927) that the maxillary stylet is 
formed in the embryo from the body of the maxilla. The latter, at the 
end of the sixth day of development, is still a simple lobe, but soon a cleft 
appears in the maxillary rudiment, which separates the basal part from 
the rest, and later from this basal part the stylet is formed as an out- 
growth. Retractor muscles from the head wall (D, rstl) become inserted 
on the bar (for) connecting the stylet with the lateral plate, and pro- 
tractor muscles (pstl) are developed in the tissue between the bar and the 
inner face of the plate. The lateral part of the maxilla bearing the palpus 
is clearly the stipes, which terminates in a single lobe (mxl) representing 
the fused galea and lacinia. The stylet, according to Reyne, is of the 



328 


PRINCIPLES OF INSECT MORPHOLOGY 


nature of a large cuticular spine produced from the mesal part of the 
maxillary base, secondarily split off from the region of the cardo and 
stipes. 

The structure of the thrips maxilla, as we shall presently see, is 
almost an exact duplicate of that of the hemipterous maxilla (Fig. 
181 B), in which the part representative of the stylet is usually drawn 
out into a long slender bristle (MxB). Furthermore, the “mouth 
forks” of the Psocidae and similar rods associated with the maxillae in 
some of the Mallophaga appear to be structures analogous at least with 
the thysanopterous maxillary stylets. All these anomolous organs 
Hansen (1930) would derive from the superlinguae and not from the 
maxillae. Hansen, however, entirely ignores the import of Reyne’s 
studies of the development of the mouth parts in the Thysanoptera, and 
he gives no weight to Reyne’s assertion that the maxillary stylets are 
actually split off from the maxUlary rudiments during embryonic growth. 

The Labium and Hypopharynx. — The labium of the Thysanoptera is 
a wide triangular appendage forming the posterior surface of the beak 
(Fig. 176 A, Lh). Distally it bears a pair of short two-segmented palpi 
and terminates in one or two pairs of flaps which are evidently the glossae 
and paraglossae. The hypophar 3 mx (Hphy) is a short medium lobe 
arising from the anterior surface of the base of the labium. Between 
it and the labrum is the food meatus of the beak (fm) leading to the 
mouth aperture (mth) behind the base of the labrum. The duct of the 
labial glands {SID) opens posterior to the base of the hypopharynx, and 
the salivary liquid is conveyed to the tip of the beak through a 
channel, the salivary meatus (sm), between the hypopharynx and the 
labium. The salivarium into which the salivary duct opens has muscles 
inserted upon it, forming thus a structure suggestive of the more highly 
evolved salivary syringe of the Hemiptera. 

The Sucking Pump. — The sucking organ of the thrips (Fig. 176 A, 
Pmp) is very similar to that of the Hemiptera (Fig. 179, Pmp). It is a 
cibarial chamber enclosed vdthin the head, and its external aperture 
(mth) is directly continuous with the food meatus (fm) of the beak. 
On its dorsal wall are inserted long bundles of dilator muscle fibers (dlcb) 
that take their origin on the anterior part of the facial region of the head 
capsule. The cranial sutures are obsolete in the Thysanoptera, but it is 
eiident that the head area on which the pump muscles arise (Clp) 
corresponds to the enlarged clypeal plate in the head of a cicada (Fig. 
177 B, Clp) or a psocid (A, Clp). 

9. THE FEEDING MECHANISM OF HEMIPTERA (RHYNCHOTA) 

The tj’pical hemipterous feeding mechanism differs from that of the 
Thjsanoptera in the following respects: (1) The beak is usually long and 



THE ORGANS OF INGESTION 


329 


slender and consists principally of the labium, which lacks palpi and 
terminal lobes, the short labrum covering only its basal part; (2) both 
mandibles are symmetrically developed, and their apical parts are drawn 
out into long slender bristles, each movable by retractor and protractor 
muscles; (3) the lateral plates of the maxillae are mostly incorporated 
into the head capsule with only their terminal parts free, maxillary 
palpi are lacking, and the maxillary stylets are long slender bristles 
similar to the mandibular bristles; (4) the salivary syringe is a weU- 
developed force pump for ejecting the saliva, and its duct transverses the 
short hypopharynx to its tip; (5) both the food canal and the salivary 
canal of the beak lie between the closely apposed inner surfaces of the 
maxillary bristles. 

The Structure of the Head. — In a typical homopterous insect, such 
as the cicada, the head capsule presents anteriorly a prominent convex 
plate (Fig. 177 B, Clp). This plate clearly belongs to the clypeal region 
of the head, since the dilator muscles of the sucking pump (cibarium) 
take their origins upon it. The plate is bounded laterally and dorsally 
by a deep groove (es) identified as the epistomal suture by the presence 
of the pits of the anterior tentorial arms in its lateral parts (Figs. 177 A, 
178, at). The epistomal suture is strongly arched upward on the face, 
its transverse dorsal part lying between the bases of the antennae. A 
comparison of the cicada head (Fig. 177 B) with the head of a psocid (A) 
leaves little doubt of the homologies of the facial plates in the two insects. 
The frons of the cicada is reduced to the small, imperfectly demarked, 
triangular area {Fr) on the top of the head, immediately above the large 
clypeal plate, bearing the median ocellus. Between the large clypeal 
plate and the base of the labrum (Lro) there is in the cicada a smaller 
anteclypeal plate {Acl-p), but in some of the Homoptera the separation 
between the two cl 3 rpeal areas is indistinct or absent. The labrum in the 
cicada is a small, slender, tapering lobe (Am) closely applied to the anterior 
side of the base of the beak. It is often called the “epipharynx.” 

On each side of the head below the compound eye are two lateral 
plates (Fig. 178, A, B) separated by a deep membranous groove (h), best 
seen in the soft head of a newly emerged imago (B). The anterior plate 
(B, A), termed the lorum by homopterists, might appear from its position 
to represent the mandible, since its dorsal extremity lies immediately 
beneath the root of the anterior tentorial arm {at) and its lower part is 
continuous mth the lateral wall of the hypopharynx (Hphy), but an 
identity vuth the mandible is not borne out by studies of the development 
of this plate. The sclerite, however, is commonly called the mandibular 
plate, since the mandibular bristle is articulated by a leverlike arm with 
the posterior border of its upper part (Fig. 181 A, Ivr), and the mandibular 
protractor muscles (pmdb) arise on its lower part. The second lateral 



aSO PRINCIPLES OF INSECT MORPHOLOGY 

plate (Fig. 178, B), which is continuous dorsally with the cranial wall, is 
known as the maxillary 'plate. Its upper part is probably the gena, but 
its lower extremity (B, Mx) and the small tapering appendicular lohe 



suspen ed from it {mxl) have been shown from embryological evidence 
^ 3e t le basal part of the maxilla fused with the lateral wall of the head. 

bristle is articulated by a lever with the maxillary plate 

, ^ maxillary protractor muscles (pmxb) arise on the 

lower part of the latter. 









THE ORGANS OF INGESTION 


331 


The posterior part of the cranium is imperfect in the cicada, and the 
labium is suspended from a large membranous ventral area continuous 
with the neck. 

It is not difficult to identify the cranial areas or sclerites of other 
Homoptera with those of the cicada if the criterion of muscle attach- 
ments is consistently followed. The clypeus (or postclypeus) is in all 
cases the plate on which arise the dilator muscles of the sucking pump. 
In Cicadellidae (Fig. 177 C), Cercopidae (E), and Membracidae (G), it 
has a position similar to that in the cicada; in the cercopids its upper 
part is reflected beyond the fastigial angle of the cranium and appears as 
a small sclerite in the dorsal wall of the head (D, Clp). The anteclj^eus 
is not always distinctly separated from the postclypeal area (E), and in 
some Fulgoridae the clypeus is undivided (F, Clp). The clypeus of the 
fulgorids is relatively small and has a ventral (F) or posteroventral 
position on the under side of the head (H) ; the vertex, on the other hand, 
is large and often elongate on the facial aspect of the head (F, F.t), 
in the lanternflies (Laternaria) it attains an extreme size and a highly 
grotesque form (H). In the Psyllidae the head is markedly opisthog- 
nathous, the clypeus being ventral, and the beak set far back on the 
under side of the head. This condition is still more exaggerated in the 
flattened “sternorhynchous” larval and adult female Coccidae. On 
the sides of the homopterous head the mandibular and the maxillary 
plates are in most cases easily recognized (C, E, G, A, B) or can be 
identified respectively as the areas on which arise the mandibular and 
maxillary protractor muscles. 

In the Heteroptera the head differs in several respects from that 
of the Homoptera, and the homologies of its parts are more difficult to 
determine. The beak usually arises anteriorly (Fig. 177 K), the clypeal 
area is typically dorsal (J, K, Clp), and there is a large ventral area 
of the head behind the beak, walled by a sclerotic hypostomal bridge 
(usually called the “gula”) between the base of the labium and the 
foramen magnum. In Corixidae and Notonectidae, however, the mouth 
parts are ventral and the facial region is directed forward (I). The area 
of the clypeus is marked by the origins of the dilator muscles of the suck- 
ing pump (Fig. 184 A, dlch) and may extend far back on the dorsal surface 
of the head (Fig. 177 J, Clp), but its upper or posterior part is not defined 
by a suture (I, J). On the other hand, the distal part of the clypeus or 
anteclypeal region (J, Aclp), called the tylus by heteropterists, is margined 
by deep clefts that separate it from lateral lobes of the head {A, A) 
known as the juga. These paraclypeal lobes appear to be the mandibular 
plates of the Homoptera, since the mandibular bristles are articulated 
to their lateral margins and the mandibular protractor muscles arise 
upon them. In Corixidae and Notonectidae the mandibular muscles 



332 


PRINCIPLES OF INSECT MORPHOLOGY 


arise on the inflected mesal margins of the lobes, which support the 
sucking pump. The maxillary plates generally have the usual position 
on the sides of the head (K, B), but in Notonectidae they are inflected 
and mostly concealed at the base of the labium. The heteropterous 
labrum is relatively long (K, Lm), in Notonectidae it is a large triangular 
flap. 

The Beak. — ^The t3rpical hemipterous beak is formed principally of 
the slender, segmented, but usually rigid labium (Fig. 178 A, Lb), which, 
in the cicada, hangs freely from the neck membrane behind the lower 



Flo. 178. — Head and beak of Magicicada septendecim. A, bead of fully matured imago. 

B, soft head of imago emerging from nymphal skin, with parts separated. 

extremities of the maxillary plates. The basal part of the beak, however; 
includes the short labrum (Lm), and the lateral spaces between the 
labrum and the labium are closed by the terminal lobes of the maxillary 
plates (mxl). Within the beak are enclosed the mandibular and maxil- 
lary bristles (B, MdB, MxB), which lie in a deep groove of the anterior 
surface of the labium (Fig. 182 A). The tip of the hypopharynx (Fig. 
178 B, H-phy) projects into the proximal part of the beak between the 
bases of the bristles, which issue from pouches of the head wall Lnvaginated 
between the sides of the hypopharynx and the inner walls of the maxillary 
plates (Fig. 180 A). 

The Hypopharynx.— The hypopharynx of the cicada (Fig. 178 B, 
Hphy) is a median conical lobe of the ventral wall of the head between the 
lower ends of the mandibular plates (A), where in the normal condition 
it is entirely concealed by the approximation of the surrounding parts (A). 
The anterior surface of the hypopharynx is continued dorsally into the 




THE ORGANS OF INGESTION 


333 


posterior (ventral) wall of the chamber of the sucking pump (Fig. 179 A, 
B, Pm!)). Within the hypopharyux is located the salivary S 3 Tinge 
{Syr), the terminal duct of which (B, sm) opens on the tip of the hypo- 
pharynx (Figs. 179 B, 180 B, SIO). The sides of the hypopharynx are 
prolonged upward as two flat, strongly sclerotized plates (Fig. 180 A, B, 
hpl) forming the inner walls of the bristle pouches {bp). The upper 
extremities of these plates are secured to the posterior transverse bar of 
the tentorium {Tnt), the union being so close in the cicada as to make it 
appear that the plates are united with the tentorial bar. 



Fig. 179. — The sucking pump and salivary syringe of Magicicada septendedm. A, 
section of the head showing position of the sucking pump (cibarium) with dilator muscles 
arising on the clypeus. B, section through the mouth region, showing food meatus C/w), 
suck-pump {Pmp), and salivary syringe {Syr). 


The Mouth, — In the soft immature stage of the cicada imago newly 
emerged from the nymphal skin (Fig. 178 B), there is seen to be a wide, 
open, transverse cleft between the base of the anteclypeus {Adp) and 
the hypopharynx {Hphy), which exposes the chamber of the sucking 
pump {Pmp). In the fully matured insect the lips of this cleft are always 
tightly shut by the contact of the anteclypeus against the lower parts 
of the mandibular plates (A), and by the closure of the epipharyngeal 
wall of the anteclypeus upon the anterior surface of the hypophar 3 mx. 
But the surface of the hypopharynx that is thus covered by the epi- 
pharynx contains a median groove, and this groove, converted into a 
tube (Fig. 179, fm) by the overlying epipharyngeal wall, remains as the 
only entrance into the pump chamber and becomes thus the fundional 
mouth. The chamber of the sucking pump of the Hemiptera, however, as 
we shall presently see, represents the preoral cibarium of orthopteroid 
insects (Fig. 155, Cb). The true mouth, therefore, is the posterior opening 
of the pump into the stomodaeum (Fig. 179 B, Mth). 



334 


PRINCIPLES OF INSECT MORPHOLOGY 


The Sucking Pump. — The narrow, tubular functional mouth of the 
Hemiptera, or channel between the anterior surface of the hypopharynx 
and the apposed epipharyngeal surface of the anteclypeus (Fig. 179 A, 
on the one hand, connects with the food canal between the max- 
illary bristles (B,/c) and, on the other, leads into the cavity of the sucking 
pump {Pmp). The latter in the cicada is a large oval chamber lying 
almost vertical in the lower part of the head (A). Its posterior and 
lateral walls are convex and strongly sclerotized. The anterior wall is 
flexible and is deeply invaginated into the lumen of the chamber. On 
its midline are inserted the converging ends of two large groups of muscle 
fibers (A, B, dlch), which have their origins on the entire inner surface of 
the postclypeal plate of the head wall (A, Clp). These muscles are the 
dilators of the pump. Their contraction lifts the infolded anterior wall 
of the organ, thus creating an upward suction through the tubular 
entrance to the chamber; with the relaxation of the muscles, the lifted 
wall springs back into the lumen by the force of its own elasticity, its 
lower end descending first. By this mechanism the food liquid is drawn 
into the pump chamber from the food canal of the beak (B, fc) and is 
expelled upward into the anterior part of the stomodaeum. 

The pump chamber of the Hemiptera very evidently represents the 
preoral cibarium of more generalized insects (Fig. 155, Ch), though it 
has usually been referred to the buccal cavity or to the pharynx. Its 
floor is formed by the proximal part of the anterior surface of the hypo- 
pharynx, and its roof is the epiphar 3 mgeal wall of the anteclypeus. In 
the cicada the lateral walls of the pump are deeply cleft by the wide 
opening between the hypopharynx and the epipharyngeal surface, as seen 
in the newly emerged imago (Fig. 178 B), and it is only in the mature 
condition that the pump cavity is concealed by the firm closure of these 
opposing parts. The true mouth of the insect is the inner opening of the 
pump (Fig. 179 B, Mtli) into the stomodaeum. 

The stomodaeum of the cicada extends upward from the inner mouth 
of the pump in the usual fashion and enlarges into a small sac (Fig. 

179 A, B, Phy) resting upon the transverse bar of the tentorium (Fig. 

180 A). This sac is the true pharynx, as shown by the fact that the 
frontal ganglion lies on its anterior end. The walls of the pharynx are 
muscular, and the organ is pro\dded with dilator muscles arising on the 
postocular region of the head and on the tentorium. Following the 
phar 3 mx is a long tubular oesophagus (Fig. 179, Oe). 

The prototype of the hemipterous sucking pump is evidently present 
in the Corrodentia (Copeognatha), where, as shown by Weber (1933, 
Fig. 56), the ingestive apparatus includes a pumplike mechanism provided 
with huge dilator muscles arising on the large postclypeal plate that 
forms most of the facial area of the head. Though the psocid pump is 



THE ORGANS OF INGESTION 


335 


attributed by Weber to the “pharynx,” it is clear from its relations to 
surrounding parts, and by its clypeal musculature, that it belongs to the 
cibarial region of the preoral cavity lying proximal to the molar surfaces 
of the closed mandibles and thus corresponds to the mouth pump of the 
Dyiiscus larva, which Weber refers to the cibarium (Ahmdhdhle). The 
position of the frontal ganglion in the cicada on the muscular pharyngeal 
sac leaves no doubt that the sucking pump of the Hemiptera is a pre- 
pharyngeal structure. The wholly nonmuseular walls of the pump and 
the origin of the dilator muscles on the clypeus attest that the pump has 
been evolved from the preoral cibarium. The enlargement and dorsal 
or posterior extension of the clypeus in Corrodentia, Thj’-sanoptera, and 
Hemiptera is clearly correlated with the great development of the 
cibarium and its dilator muscles. 

Tnt dlsyr 


B 

Fio. 180. — The salivary syringe and associated structures of Magicicada septendecim 
A, posterior vierv looking into back of head between maxillary plates. B, diagrammatic 
cross section through the maxillary plates (B), bristle pouches (.bp), hypopharynx, and 
salivary syringe. 

The position of the sucking pump within the head varies considerably 
in different groups of Hemiptera. In the Heteroptera the organ generally 
lies farther back than in the Homoptera and has a more horizontal 
position (Fig. 184 A). Its various types of structure and many details 
in the mechanism of the sucking apparatus have been admirably por- 
trayed by Weber (1928, 1928a, 1929, 1930, 1933). 

The Salivary S 3 rringe. — The salivary syringe of the Hemiptera is a 
small, hollow, cup-shaped organ (Fig. 179 B, Syr), which at its distal 
end receives the common duct of the salivary glands {SID), and dis- 
charges to the exterior through an outlet tube {sm) opening on the tip 
of the hypopharynx {Hphy). The udder inner end of the cup is deeply 
invaginated and supports a short apodeme on which are inserted a pair 
of large muscles {dlsyr) arising on the mesal surfaces of the long lateral 
plates of the hypopharynx (Fig. 180 B). The mechanism of the appa- 
ratus is very simple : the contraction of the muscles lifts the invaginated 





336 


PRINCIPLES OF INSECT MORPHOLOGY 


end wall of the cup, and the latter springs back by its own elasticity when 
the muscles relax. Thus the salivary liquid is drawn into the chamber 
of the cup through the salivary duct and is forcibly expelled through the 
outlet tube. In some of the Hemiptera at least, as described by Weber 
(1930), the entrance and exit of the pump chamber are provided with 
vahnilar flaps to prevent the backward flow of the liquid. Though the 
organ is commonly known as the “salivary” pump, or syringe, the secre- 
tion of the connected glands, which undoubtedly are the homologues of 
the labial glands of other insects, probably does not have in all Hemiptera 
a strictly digestive function. 

Structurally the salivary syringe of the Hemiptera is very similar to 
the corresponding organ of Diptera; in each case the pump chamber is 
provided with hypopharyngeal muscles only, and the exit duct traverses 
the hypopharynx to open on the tip of the latter. Morphologically there 
can be little doubt that the syringe is a highly specialized development 
of the salivary pocket, or salivarium, of orthopteroid insects (Fig. 155, 
Slv). Its dilator muscles are the dorsal hypopharyngeal muscles of the 
salivarium (Is). Representatives of the ventral labial muscles (2s, 3s) 
are absent in both Diptera and Hemiptera. There is no evidence to 
show' how' the s 3 Tmge and its exit duct have become enclosed within the 
hypopharynx; but if the duct represents the salivary meatus of general- 
ized insects (Fig. 155, sm), it is perhaps possible that the apparent 
posterior w'all of the hypopharynx in the Hemiptera is a fold of the labial 
w'all. On the other hand, the whole apparatus may be simply infolded 
within a closed groove of the hypopharyngeal wall. It is interesting to 
observe in this connection the much more primitive structure in the 
Thy'sanoptera (Fig. 176 A). 

The Mandibular and MaxiUary Bristles. — ^The long bristle-likc 
stylets characteristic of most of the Hemiptera arise from the walls of the 
bristle pouches, W'hich, as already noted, are invaginations of the ventral 
wall of the head betw'een the inner surfaces of the maxillary plates and 
the outer surfaces of the hypopharyngeal plates (Fig. 180 B, hp). Emerg- 
ing from the pouches the bristles converge along the sides of the hypo- 
phar 3 mx (A, Hphy), and, as they enter the groove of the labium, they 
become adherent to one another in a compact bundle, or fascicle (Fig. 
182 A). The bases of the bristles are enlarged; those of the mandibular 
pair lie anteriorly in the pouches, those of the maxillary pair posteriorly. 
Within the labium, the mandibular bristles are the outer pair of the 
fascicle {MdB), the maxillary bristles the inner pair {MxB). 

The mandibular bristles of the cicada are slightly thicker than the 
maxillarj' bristles. The enlarged base of each lies in the bristle pouch 
just behind the lower end of the corresponding mandibular plate (Fig. 
181 A) and is produced proximally into tw'o long arms. One arm (ra) 



THE ORGANS OF INGESTION 


337 


proceeds dorsally in the inner wall of the pouch (bp) and gives attach- 
ment to the retractor muscles (rmdb) arising on the dorsal wall of the 
head. The other arm (Ivr) goes dorsally in the external membranous 
groove (Fig. 178 B, A) between the mandibular and maxillary plates on 
the side of the head, and its upper part bends forward to articulate with 
the dorsal end of the mandibular plate (Fig. 181 A, g). This arm sup- 
ports for most of its length a wide, thin apodemal inflection (ap), on 
which are inserted the protractor muscles of the bristle (pmdb), which 
have their origins on the inner face of the mandibular plate (A). The 
protractor arm of the mandibular bristle (Ivr) thus functions as a lever, 
and its relations to the mandibular plate and the base of the bristle are 



Fig. 181. — Motor mechanism of the mandibular and maxillary bristles of Magicicada 
scptendecim. A, base of a mandibular bristle with retractor {rmdb) and protractor 
{pmdb) muscles. B, base of left maxillary bristle, posterior view, with retractor and 
protractor muscles. 

very similar to those of the maxillary lever (B, Ivr) to the maxillary plate 
and the base of the maxillary bristle. The mandibular lever is differ- 
entiated from the mandibular plate during the transformation from the 
nymph to the imago. 

The maxillary bristles arise from the walls of the bristle pouches at 
a higher level than do the mandibular bristles. Upon the base of each 
(Fig. 181 B, MxB) are inserted the retractor muscles (rmxb), which 
arise on the dorsal walls of the head, and also a large protractor muscle 
(tp77ixb) having its origin ventrally on the inner face of the maxillar 3 ’' 
plate (B). In addition to these muscles there is a second set of protractor 
fibers (2p77ixb) arising on the maxillarj’’ plate and inserted on the leverlike 
sclerite (Ivr) that lies in the wall of the bristle pouch and connects the 
base of the bristle vuth the posterior edge of the maxillary plate. 

The two sets of bristles extend out of the bristle pouches along the 
sides of the hypopharynx, where the maxillarv bristles shde upon track- 



338 


PRINCIPLES OF INSECT MORPHOLOGY 


like ridges of the lateral hypopharyngeal plates. Beyond the tip of the 
h 3 T 3 ophar}Tix the bristles turn downward to enter the groove of the 
labium and those from opposite sides converge, the maxillary bristles 
becoming here interlocked, while the mandibular bristles take then- 
positions at the sides of the maxillary bristles. In the groove of the 
labium the four bristles are thus assembled in a slender fascicle. The 
maxillary bristles form the core of the fascicle (Fig. 182, MxB) with their 
inner faces closely applied to each other and usually held firmly together 
by dovetailing grooves and ridges extending throughout their lengths (C). 
In some species of Hemiptera the mandibular bristles are similarly locked 
to the maxillary bristles (C). Between the maxillary bristles there are 
two minute tubular canals formed by opposing grooves on the inner 
surfaces of the bristles (B, C, /c, sc). The position of these canals is 



Fig. 182. — Sections of the beak and feeding bristles of Hemiptera. A, Magicicada 
seplcndccim, labium and bristles. B, Aphis rumicis, labrum, labium, and bristles. {From 
Datidson, 1925.) C, Anasa tristis, bristle fascicle. {From D. G. Tower, 1914.) D, Cimex 
leclularius, bristle fascicle. {From Kemper, 1932.) 

such that, where the bristles diverge to enter the bristle pouches, the 
anterior canal (Fig. 179 B,/c) opens into the mouth channel (/m), and the 
posterior canal (sc) receives the tip of the h3T5opharynx, on which is 
located the aperture of the salivary meatus {sm). The anterior canal 
is, therefore, the food canal (/c), and the posterior one the salivary canal 
(sc). In the distal part of the labium the bristle fascicle may become 
twisted, with a consequent change in the relative positions of the canals. 

The Labium, — The labium of the Hemiptera, when well developed, 
is a long, slender, rigid organ divided into three or four parts, or “seg- 
ments” (Fig. 179, Lb). Its anterior surface is deeply concave to form 
the channel of the beak containing the mandibular and maxillary bristles 
(Fig. 182 A, Lb). The morphology of the hemipterous labium is not 
understood. The cranial muscles that move it are inserted on the first 
or second segment, and it would seem, therefore, that the principal part 
of the labium of the Hemiptera consists of the prelabium alone, the post- 
labium being represented by a basal segment or by the ample membranous 
area at the base of the organ. The cranial muscles act as either retractors 
or protractors according to whether they are inserted directly on the 
labial base or on an apodemal arm of the latter. The interior of the 


THE ORGANS OF INGESTION 


339 


labium contains an elaborate musculature, which has been fully described 
in Aphis fabae by Weber (1928a) and in Cimex by Kemper (1932). 

The Piercing Mechanism. — In most of the Hemiptera, and probably 
in all members of the order, as has been shown by Weber (1928, 1930), 
the mouth bristles are not moved by simultaneous contractions of their 
muscles. The mandibular bristles are the chief piercing organs. When 
the insect begins an insertion of its bristle bundle (Fig. 183, 1), one 
mandibular bristle is thrust out a short distance in advance of the 
other to puncture the food tissue (2), and then the opposite mandibular 
bristle is protracted until its tip meets that of the first (3). Now 
the two maxillary bristles are lowered together until their tips lie 
between those of the two mandibular bristles (4). At a single thrust 
a bristle is extruded no farther than the maximum distance the short 
protractor muscle can drive it with one contraction. This distance at 

f-i4| 

1 2 3 4 5 6 7 8 

Fio. 183. — Successive stages in the insertion of the feeding bristles of Hemiptera. (Dta- 
fframs based on figures from TVcber, 1928.) 

best is insignificant compared with the depth to which the bristle bundle 
can finally be sunken into the food tissue. Repeated thrusts, therefore, 
are necessary (5 to 8). But a repetition of the insertion process necessi- 
tates that the protracted bristles be in some way secured in the new 
position in order to resist the backward pull of the retractor muscles 
that restores the protractors to their functional lengths. In some cases 
the bristles are anchored in the food tissue by barbs on their tips; in 
others they are held in a clasp of the enclosing labium. 

^^Tien the mouth bristles are not in use they do not normally protrude 
from the tip of the labium. With most species, moreover, the bristles 
are not long enough to be projected from the labium except for the verj’’ 
short thrust given them by the protractor muscles, and in such cases 
the exposure and insertion of the distal part of the bristle fascicle are 
made possible by a retraction or folding of the labium that does not 
involve the bristles. 

In homopterous forms ha%'lng mouth bristles of usual length, the 
labium is suspended from a membranous area of the head and is often 
flexible at its base (Fig. 184 C). The e.xposure of the bristles in such 
species is brought about bj’’ a retraction of the labium or bj’’ a backward 





340 


PRINCIPLES OF INSECT MORPHOLOGY 


folding of its basal segments, allowing the head to be lowered as the 
bristles penetrate the food tissue (D). In some species of aphids an 
indi\idual in the act of inserting its bristles stands high on its front legs 
and plants the beak vertically against the leaf (Fig. 185 ) ; as the bristles 
sink into the leaf tissue, the body is lowered anteriorly and the basal 
part of the labium bends back like the elbow of an arm, while the terminal 
part retains its grasp on the bristles; finally, when the bristles are in at 
full length, the insect stands almost on its head. In the adults of 
Aleurodidae, according to Weber (1928), the labium is equipped with 
protractor muscles; this provision allows these insects to make a quick 
departure from a feeding puncture, but the aphids, which have no pro- 
tractor mechanism for the labium, often have much difficulty in extracting 
the mouth bristles. 



Fio. 184. — Various positions of the hemipterous labium during feeding. A, Grapho- 
soma italicum. {From Weber, 1930.) B, Cimex. {From Kemper, 1932.) C, D, Trialeu- 
rodcs, before and after insertion of the bristles. {Adapted from Weber, 1928.) 

With many of the Heteroptera the long rigid labium is firmly articu- 
lated to the head somewhat behind the exit of the bristles from the 
latter, and in such species it is probable, as shown by Weber (1928), 
that a preliminary exposure of the tips of the bristles is effected merely 
by the forward svdng of the beak from its horizontal position of repose. 
The further exposure and the insertion of the bristles are usually accom- 
panied by an elbowlike bend of the labium between the first and second 
segments (Fig. 184 A), while the base of the bristle fascicle (5/) is held 
in the groove of the labrum (Lm). In the bed bug, however, according 
to Kemper (1932), the labium bends between the third and fourth 
segments (B) and is further shortened by an invagination of its base 
into the head, and to a smaller degree by a telescoping of its segments. 

It is evident, now, that such simple devices as those just described 
for the exsertion of the mouth bristles can give effective service only to 
larger species or to species that obtain their food but a short distance 
below tile surface of the food tissue. Very small sucking insects or those 



THE ORGANS OF INGESTION 


341 


that draw their food from relatively greater depths must have proportion- 
ately long mouth bristles. Such species, therefore, are confronted with 
the problem of storage for bristles often of greater length than the 



Fig, 185. — Attitudes of an aphis during feeding. 


body, and with that of exserting the bristles far beyond the tip of the 
labium. 

Hemiptera with bristles much longer than the labium include the 
larvae of Psyllidae and Aleurodidae, larvae and adult females of 
Coccidae, the Coptosmatidae, and the Aradidae. The problem of bristle 
storage has been solved by these insects in different ways. With the 



Fig. ISO. — Structural details and various de^■ices for the storage of the mouth bristles 
of long-bristled Hemiptera. A, Psylla mali larva, bristles looped outside of head. B, 
Pscudococcus adult female, bristles in crumena. C, Tropidotylus fasciolatus, bristles in 
crumena. D, Aradus, bristles coiled in preoral ca^•ity. E, Bozius respersus, bristles 
looped in base of labium. F, Pscudococcus, labium and bristle clamp. G, Psylla mali, 
bristle clamp of labium. (A, B, F, G from Weber, 1928; D from Weber, 1930; C, P, from 
China, 1931.) 


larval psyllids the bristle fascicle when retracted is projected forward 
from the base of the labium in a large free loop beneath the head (Fig. 
186 A). In the Coccidae and the larvae of Aleurodidae the retracted 
fascicle is received into a long internal pouch, the crumena, e.xtending 


342 


PRINCIPLES OF INSECT MORPHOLOGY 


backward from above the base of the labium into the thorax (B, Cm). 
In some members of the heteropterous family Coptosomatidae (Plata- 
spididae), as shown by China (1931), the retracted bristle fascicle is 
looped posteriorly in a large membranous diverticulum at the base 
of the second labial segment (Fig. 186 E), while in others (C) the fas- 
cicle is received into a long crumenal sac {Cm) resembling that of the 
Coccidae, extending from the head into the base of the abdomen. The 
coptosomatids are mostly fungus feeders, and the length of the mouth 
bristles, as suggested by China, is probably an adaptation for probing 
lengthwise through the mycelial filaments. In the Aradidae, finally, 
which are also fungus feeders, the retracted bristle fascicle is coiled in a 
large chamber of the preoral ca\dty anterior to the mouth of the sucking 
pump (Fig. 186 D). 

The means by which the long-bristled Hemiptera, particularly the 
minute Coccidae, are able to protrude their threadlike mouth bristles 
from the head and to insert them into woody tissues was for a long time 
an outstanding entomological mystery. Some writers attempted to 
exT^lain the exsertion of the bristles as brought about by a muscular 
contraction of the crumena, and others postulated blood pressure against 
the sac as the active force, but these theories could not apply to larval 
Psyllidae with the bristles looped outside the head, nor would they in any 
case account for the retraction of the bristles. A consistent and con- 
\ancing explanation of the mechanism of protraction and retraction 
of the mouth bristles in these species, however, has recently been given 
by Weber (1928, 1930, 1933), and the following descriptions are based 
on his observations. 

Three anatomical facts explain the principle by which the mechanism 
of exsertion and retraction accomplishes its results. First, the protractor 
and retractor muscles are able to move the bristles but a very short 
distance with each contraction; second, the four bristles are firmly 
interlocked in the fascicle but slide freely upon one another; third, there 
is some provision for holding the bristles in place, after each protraction 
or retraction, that prevents the antagonistic muscle from undoing the 
work of the other. The holding apparatus in the Psyllidae, Aleurodidae, 
and Coccidae is a clamp in the labium, consisting of a narrowed and 
strongly sclerotized area in the labial groove with muscles to regulate 
its pressure on the bristle fascicle (Fig. 186, F, G). In other families the 
same effect is accomplished by barbs on the ends of the bristles. 

The musculature of the mouth bristles is mechanically the same 
in all cases, and the alternating thrusts and pulls are exerted on the 
several bristles of the fascicle in the manner already described for the 
Hemiptera in general (Fig. 183). The only difference in the long- 
bristled forms is that the retracted fascicle is thrown into a loop or coil 



THE ORGANS OF INGESTION 


343 


somewhere between its base and its cxtremit 3 " (Fig. 187). The loop, 
however, makes no difference in the movement of the l)ristlps, because 
the latter are securely held together bj'- interlocking grooves and ridges 
and slide freely on one another. The successive contractions of the 
protractor muscles have no effect on the loop (1 to 4), the bristles being 
moved alike at both ends. But, after each thrust, when the fascicle is 
held in place by the labial clamp, the simultaneous contraction of the 
retractor muscles takes up a little of the slack in the loop (5). Hence 
the bristles penetrate deeper and deeper with the succeeding outward 
thrusts, while the series of pulls on their bases is expended against the 



t- ; LJ LJ LJ L 






Fig. 1S7. — The mechanism of insertion of the feeding bristles by Hemiptern ha\'ing 
long bristles stored in a loop or coil -svhen retracted. {From Weber, 1933.) 1, beak 

placed against the plant surface. 2, 3, 4, first insertion of the mandibular and maxillary 
bristles, as in Fig. 1S3. by contraction of the protractor muscles. 5, fascicle of bristles 
held in labial clamp while loop shortened by contraction of the retractor muscles. 0, 
bristles inserted full length after successive repetitions of movements 2 to 5. 


loop, with the result that the latter is gradually shortened, until it is 
obliterated when the bristles are exserted at full length (6). The looping 
of the fascicle during retraction of the bristles requires only a reverse 
action of the labial clamp. 

It still seems almost bejmnd belief that the delicate bristles of such 
small insects as Coccidae can penetrate the bark of trees; but since it is 
an observable fact that thej’’ do so, the feat evidentl}’’ is not impossible. 
It is known, however, that the salivarj’- secretion of some Hemiptera 
has a solvent effect upon plant tissues and thus facilitates the insertion 
of the bristles. The salivar}^ canal of the beak, it should be recalled, 
accompanies the food channel throughout the length of the bristle 
fascicle. 


344 


PRINCIPLES OF INSECT MORPHOLOGY 


10. THE FEEDING MECHANISM OF ANOPLURA 

The feeding equipment of the true lice is a highly specialized piercing 
and sucking mechanism. The morphology of the piercing organs is not 
definitely known, and observations on the structure of the mouth parts 
gi^■en by various investigators do not agree in all respects, though details 
have been minutely described and figured. Our present information 
on the mouth parts of the Anoplura is contained in the work of Cholod- 
kowsky (1904), Enderlein (1905, 1905a), Pavlowsky (1906), Harrison 
(1914), Sikora (1916), Peacock (1918), Florence (1921), Vogel (1921a), 
and Fernando (1933), while summarized accounts are given by Metcalf 
and Flint (1928), Patten and Evans (1929), and Imms (1934), though 
with variations in detail according to the source selected 


FrGng- Phy Br n SoeGng- 



Fio. 188. — The piercing and sucking apparatus of Anoplura. A, section of the 
head showing buccal and pharyngeal pumps (BuC, Phy), and suboral sac {Sac) containing 
the piercing stylets {Stl). {Diagram composed from Sikora, 1916, and others.) B, section 
of the labrum and piercing stylets of Pediculus vestimenti. {From Vogel, 1921a.) 

The essential structure of the piercing and sucking apparatus of 
Pediculus appears to be as follows. The elongate head terminates 
anteriorly in a small, protractile, snoutlike tube, known as the mouth 
cone, rostrum, proboscis, or prestomum. The organ appears to be the 
labrum (Fig. 188, A, Lm). It has a terminal aperture continued into a 
median ventral cleft, and its inner walls are armed with small recurved 
teeth (b), which, when everted, enable the parasite to obtain a hold 
on the skin of its host. Other members of the mouth parts are not 
ordinarily visible externally. The ventral channel of the labrum 
leads into a tubular preoral cavity, the “buccal funnel” (PrC), in the 
anterior part of the head. The head capsule is closed below by a long 
hypostomal wall, the distal extremity of which forms the lower lip (hst) 
of the preoral cavity. From the posterior end of the preoral cavity 
the mouth (Mth) opens dorsally into a two-chambered sucking pump 
{BuC, Phy), which terminates in a slender oesophageal tube (Oe). Ven- 
t rally the preoral cavity is e.xtended below the mouth in a long sac 
(iSoc) containing a group of slender piercing organs (Stl). The first 
chamber of the sucking pump (BuC) is perhaps the buccal cavity, since 
it lies anterior to the frontal ganglion (FrGng) ; the second (Phy) is with- 



THE ORGANS OF INGESTION 


345 


out doubt the true pharjoix; both evidently belong to the stomodaeum 
since their walls have a sheath of circular muscle fibers. A more 
careful stud}" of the relations of the dilator muscles to the head wall 
and to the frontal ganglion connectives may be expected to give more 
conclusive e\’idence as to the identities of the several parts of the food 
passage. 

The piercing organs of the louse (Fig. ISS A, Stl), according to 
Vogel ( 1921 a), consist of three superposed stylets (B, Mx, Hphy, Lb). 
The stylets arise posteriorly from the walls of the containing sac (A), 
and, in the retracted condition, their distal ends e.xtend to the base of 
the labrum in the preoral cavit3’-, where they are enshcathed in folds 
of the walls of the cavity. The most dorsal stjdet appears to be formed 
of two united appendages (B, Mx), the distal parts of which have their 
free edges rolled upward to form a tubular channel (/c), which is the 
food canal serving to conduct the ingested blood from its source to 
the mouth. The intermediate st3det (Hphy) is a slender rod traveled 
b3’’ the salivaiy duct (A, SID), which opens on its extremit3\ The ventral 
st3det (B, Lb) is a broader appendage with distinct dorsal and ventral 
walls (which have been mistaken for separate pieces). The dorsal wall 
is decpl3’’ grooved b3’’ a channel containing the median st3det. Distall3' 
the ventral st3det ends in three sharp-pomted, serrate lobes, which are 
the piercing organs of the louse. The proximal ends of the dorsal and 
ventral stylets give off long apodemal arms (A), one pair from the former, 
a dorsal and a ventral pair from the latter, which are imbedded in folds 
of the wall of the sac and give insertion to protractor muscles arising 
anteriorly on the sac walls. Other muscles, arising on the head and 
inserted on the sac, serve for the retraction of the sac and the st3'lcts. 

The st3dets have been generally assumed to represent in some way 
the mouth parts of the louse. Investigators are agreed that the man- 
dibles are absent in adult Anoplura or arc reduced to a pair of small 
plates l3’ing at the sides of the preoral ca^it3^ The mandibulate elephant 
louse (Haemaioinyzus), as shown b3’- Ferris ( 1931 ), has none of the 
special features of the sucking lice and is perhaps to be classed with 
the Mallophaga. Cholodkowsk3' ( 1904 ) claimed that both the mandibles 
and the maxillae of the Anoplura disappear during embr3'onic develop- 
ment, and that the piercing organs are secondar3' structures concealed 
b3" the labium. Enderlein and Vogel, however, from anatomical studies 
of the adult insect, have contended that the dorsal stylet (Fig. ISS B, jl/x) 
represents the united maxillae, that the intermediate st3-lct (Hphy), 
traversed b3' the salivar3' duct, is the h37)ophar3mx, and that the ventral 
st3'lct (Lb) is the labium. This interpretation appears to be confirmed 
b3- the more recent stud3' of Fernando ( 1933 ) on the embr3*onic develop- 
ment of the mouth parts of Pcdicitlns humatitts. 



346 


PRINCIPLES OF INSECT MORPHOLOGY 


According to Fernando, the usual gnathal appendages appear on the 
head of the embryo of Pediculus, there being present at an early embry- 
onic stage paired rudiments of mandibles, maxillae, and the labium. The 
mandibles undergo no development and finally disappear. The maxil- 
lary and labial rudiments, however, elongate and those of each pair 
unite, forming thus two median organs which become the dorsal stylet and 
the ventral stylet. The stomodaeum is formed in the usual manner as 
a median invagination betAveen the antennae and the mandibles, and 
the labrum appears anterior to the mouth. The stylets are now Avith- 
draAvn into an inAmgination of the ventral wall of the head behind the 
mouth, and the lips of the pouch groAV out to form the enclosing sac. 
The intermediate stylet is then formed by an outgroAvth betAveen the 
bases of the maxillary and labial stylets involving the terminal part 
of the salivary duct and eAudently represents the h5rpopharynx. The 
labrum becomes the conical snoutlike rostrum embracing the tips of the 
retracted stylets. 



CHAPTER XIII 


THE ALIMENTARY CANAL 

The organs of alimentation in metazoic animals have to do ^^•itll the 
intake of raw food materials, the digestion and absorption of nutrient 
substances from these materials, the ejection of the unused residue, and 
the distribution within the body cavitj’' of the absorbed products of 
digestion to the cellular tissues where they are utilized in the processes of 
growth and metabolism. The organs of ingestion, digestion, absorption, 
and egestion are the parts of the alimentaiy canal and the digestive 
glands that pour their secretions into it. The medium of distribution 
is the blood. 

Feeding is primarily a matter of getting nutrient materials from 
the environment through the integument of the organism; assimilation is 
the utilization of the absorbed materials by the cells of the body tissues. 
Most metazoic animals in their feeding habits differ fundamentally from 
such protozoans as the amoeba in that the}-^ do not take solid particles 
of food matter through the body wall; the requisite nutrient substances 
arc dissolved in liquids thrown off from a part of the body and are then 
absorbed into the latter. The primitive stomach, or archcntcron of the 
gastrula, is simpl}’- a food pocket invaginated on one side of the body, 
the wall of which is formed of specialized digestive cells. The more 
complex alimentary canal of the higher animals, therefore, must be 
regarded as mcrelj’^ a more efficient device for holding food materials in 
proximity to a digestive and absorptive surface, to which have been added 
special mechanisms for ingestion and egestion. The lumen of the food 
tract is a part of the environment enclosed vithin the animal. 

1. DEVELOPMENT OF THE ALIMENTARY CANAL 

The embryonic development of the alimcntarj' canal, described 
in Chap. II, gives us a misleading concept of the true nature of the 
digestive tract of arthropods, espcciall}' of insects, for we arc induced 
to think of it as consisting of an cndodermal stomach formed entircl}' 
within the body, which onh' secondarily acquires openings to the exterior 
through an ectodermal stomodacum and an ectodermal proctodaeum. 
The ontogenetic development of the digestive tube, however, is clcarty 
an embryonic adaptation to the conditions of life in the egg and is not 
to be taken as a literal rej)Ctition of plndogenetic history. The mesen- 

347 



348 


PRINCIPLES OF INSECT MORPHOLOGY 


teron is the primary stomach and there is little probability that it was 
ever a elosed sac in any of the adult ancestors of the arthropods; the 
stomodacum and proctodaeum are later ingrowths of the ectoderm at 
the primitive oral and anal apertures of the mesenteron. The stomodaeal 
and proctodaeal openings into the stomach, therefore, in a sense, are the 
true mouth and anus of the arthropod, w^hich have been carried inter- 
nally by an inward growth of the circumoral and circumanal parts 
of the ectoderm. According to Henson (1931), the innermost cells of 
the stomodaeum and proctodaeum in lepidopterous larvae form interstitial 
rings of ectodermal cells that retain the power of mitotic division and at 
the time of metamorphosis regenerate the epithelium of the stomodaeum 
and the proctodaeum. During the larval period these parts of the 
alimentary canal grow by enlargement of the epithehum cells but not by 
cell multiplication. Though the stomodaeum and proctodaeum are 
primarily organs of ingestion and egestion, they have come to serve also 
in various other capacities accessory to the function of the stomach. 

The cells of the mesenteron maintain their early acquired activities 
that particularly adapted them to the functions of digestion and absorp- 
tion. They are continually subject to disintegrating processes, and some 
of them, at least, retain the power of mitotic division to replace those 
depleted by digestive activities, or to regenerate the entire epithelium 
at the time of metamorphosis or even at the larval moults. 

2. GENERAL STRUCTURE OF THE ALIMENTARY CANAL 

Since the digestive tract is but an infolded part of the body wall, its 
own walls have the same essential structure as that of the body integu- 
ment. They consist of a layer of cells, the enteric epithelium, resting 
upon a hasement membrane turned toward the somatic cavity and lined 
internally by a cuticular intima. The intima is best developed in the 
stomodaeum and proctodaeum; in the mesenteron, if present at all, it 
has a ver}’’ delicate texture and is often disrupted by the activities of the 
epithelial cells. All parts of the alimentary canal are usually invested 
in a muscular sheath, or muscularis, derived principally from the splanch- 
nic layer of the mesoderm. Other muscles, probably of somatopleure 
origin, e.vtend from the body wall to the alimentary canal. These 
extrinsic muscles are known as the suspensory or dilator muscles, the 
second term probably better expressing their function. 

In form the alimentary canal of insects is a tube, either straight, 
or variously looped upon itself if its length exceeds that of the body. 
In its simplest development the tube shows little differentiation bejmnd 
the primary di\nsion into stomodaeum (Fig. 189, (:,lom), mesenteron (Ment), 
and proctodaeum (Proc). The functional stomach, or ventricidus, is the 
mesenteron. Usually a circular valve-like fold separates the cavities 



THE ALIMENTARY CANAL 


3-19 


of adjoining sections, that between thestomodaeum and mesenteron being 
known as the stomodacal, or cardiac, valve (SVlv), tlie one closing the 
entrance to the proctodaeum as the proctodacal, or pyloric, valve {PVlv). 

Few insects, however, have an alimentarj' eanal so simple as that 
just described. Generally each of the primarj’’ sections of the tube, 
particidarly the first and the third, arc differentiated into several more 
or less distinet regions, and diverticula of various forms grow out from 
the walls (Fig. 190). The principal outgrowths of the alimentar}’’ 
canal arc the Malpighian tubules {Mai), which are attached to the 



Fi<3. iso. — Tlio nlimontary cannl of a collombolnn, Tomocmix niocr, .showinc in pimplo 
form the primary components of the food tract without sccondnrj* specializations. (From 
Folsom and (Vcltcs, I90G.) Mail, mesenteron; Proc, proctodaeum; PVh, proctodacal. 
or pyloric, valve; Slom, stomodaeum; ST7r, stomodaeal, or cardiac, valve. 

anterior end of the proctodaeum, but various diverticula occur also on the 
mesenteron, and glands may open into the stomodaeum. The ali- 
mcntaiy canal in all its parts is subject to many variations of form in 
different insects. Some of its principal t}T)cs of structure are shown in 
Figs. 195, 196, 198, 199. During metamorphosis the entire digestive 
tract often undergoes much reconstructive alteration both in external 
form and in its histological structure, as is well illustrated in the Lepi- 
doptera (Fig. 197), the changes being adaptive to the different feeding 
habits of the j'oung and the adult of the same species. 

3. THE STOMODAEUM 

In its .simplest condition the stomodaeum is little more than an inlet 
to the stomach or a short conduit to the latter from the mouth (Fig. 189, 
Slom). In most insects, however, the stomodaeum is a long tube of 
which the middle part is enlarged to form a storage chamber for reserve 
supplies of food; and this function a.'J.sumed ly the middle region was 
evidently the jwceursor of a specialization of the fore part of the tube 
into an organ of ingestion, and of the posterior part into a “stomach 
mouth” for regulating the p.as.«age of food into the ventriculus, or even 
in some cases for giving it a second chewing. Thus the stomodaeum, or 
primitive oesophagus, h.as become differentiated into three primary 
regions, namely, the pharynx (Fig. 190, Phy), the crop (Cr), and the 
provcnlriculits (Pvcnl). An undifferentiated part of the ttibc m.ay remain 
as a definite oesophagus (Oc) between the phar^mx and the crop, and, as 



350 


PRINCIPLES OF INSECT MORPHOLOGY 


we saw in the last chapter, the initial region just within the mouth is 
often distinguishable from the pharynx as a buccal cavity (BuC). 

The primary functions of the stomodaeum thus appear to be mechan- 
ical; but there is little doubt that the organ in insects has secondarily 
come to be also a physiological adjunct to the stomach by increasing 



Fig. 190. — Diagram showing the usual subdivisions and outgrowths of the alimentarj 
canal. AInt, anterior intestine; An, anus; BuC, buccal cavity; Car, cardia; Cln, colon; 
Cr, crop; GCa, gastric caecum; Jl, ileum; Mai, Malpighian tubules; Ment, mesenteron 
(ventriculus) ; Mih, mouth; Oc, oesophagus; Phy, pharynx; Pint, posterior intestine (rec- 
tum); Proc, proctodaeum; Pvent, proventriculus; Py, pylorus; Reel, rectum (reel, rectum 
proper; rsc, rectal sac); Stom, stomodaeum; Vent, ventriculus. 


the space available for digestive purposes, since the food stored in the 
crop is subject to the action both of the salivary liquid mixed with the 
food during ingestion and of gastric juices that flow forward into the crop 
from the ventriculus. 

Histology of the Stomodaeum. — The walls of the stomodaeum in 
general have a simple structure. The epithelium (Fig. 191 A, Epth) is 



Sections of the stomodaeum of a grasshopper, Lissosteira Carolina. A, 
proventriculus. BMb, basement membrane; cmcls, circular muscles; 
Ep^. epithelium; In, intima; Imcls, longitudinal muscles; Bum, lumen; m, muscles in folds; 
Tra, trachea. 


usually flat, and the cell boundaries often indistinct. The intima {In) 
is relatively thick; its surface for the most part is sparsel}’’ covered with 
short hairs or spicules, though in the pharynx and the proventriculus 
there may bo areas closely beset with long hairs or spines. In the 
proventriculus of some insects the intima is dense and produced into 



THE ALIMENTARY CANAL 


351 


lobes and teeth forming a special armature (Fig. 194). Both the epi- 
thelium and the intima are thrown into longitudinal fold.s which in 
most parts of the slomodaeum allow for cx])ansion of the lumen as the 
latter becomes filled with food; Imt a certain number of the lobes arc 
often definite structures, as shown bj' the increased thickne.«s of the 
intima covering them and of the underlying epithelial cells (Fig. 191 B). 
These definite folds usuallj’ occur in multiples of two or three, there 
being commonly four, six, or eight major folds, with the same or a greater 
number of minor intermediate folds between them. The major folds 
are particularl}’^ developed in the phar 3 ’ngeal and proventricular regions. 

The muscular sheath is a ver}' important part of the stomodaeum. 
It consists in general of an outer lajw of circular fibers (Fig. 191 A, 
cmcis) and of an inner la 3 ’er of longitudinal fibers (/wc/.<?); but a detailed 
stud 3 ' of the stomodacal muscularis .shows that its fibers do not nece.s- 
sarily adhere strictl 3 ' to the t 3 'pical arrangement in all parts of the 
stomodaeum. The circular fibers gcnerall 3 ' run continuousl 3 ' around 
the tube without attachments to the latter. The longitudinal muscles, 
on the other hand, arc sometimes inserted on the intima in the same 
manner as the somatic muscles are attached to the bod 3 ' wall, but in 
other cases the 3 ' too appear to have no connections with the intima or 
the epithelium and arise as confluent branches of the circular muscles. 
The last condition is well shown in the crop of a caterpillar (Fig. 106, Cr) 
where the muscularis forms a veritable plc.xus of branching and uniting 
fibers constituting a sheath about the inner walls of the tube, but having 
no intimate connections with the latter. Wlicre the folding of the 
stomodacal walls is pronounced, the longitudinal muscles tend to become 
grouped in the spaces of the folds (Fig. 191 B). 

The stomodaeum is gencrall 3 ' well provided with dilator muscles 
(Figs. ICO, 193). These muscles take their origins on the walls and 
apodemes of the head and on the walls of one or more of the thoracic 
segments. Their central ends usuall 3 ' penetrate between the fibers of 
the muscularis to be inserted either on the stomodacal ejiithelium or 
on the intima, but in certain cases some of their fibers appear to unite 
with those of the muscularis. 

The Buccal Cavity. — The true buccal cavit 3 ' of the insect is the 
oral part of the stomodaeum (Figs. 155, 190, 192, 193, BiiC) and .should 
not be confused with the ])reoral cavit 3 ' (Fig. 155, PrC), or external spare 
enclosed between the mouth parts, which is often incorrecth' called the 
“mouth ca\"it 3 '.” As shown in the last chapter, the buccal cavil 3 * 
u.sualh' is not structural^'' difTerentiatcd from the phar 3 'nx (Fig. 192), but 
it ma 3 ' be defined as the initial part of the stomodaeum on 'vhich arc 
in.'^erted the second grouj) of dilator mu.«cles taking their origins on the 
clvpcus, or the ch'jieal area of the head, and having their insertions 



352 


PRINCIPLES OF INSECT MORPHOLOGY 


anterior to the frontal ganglion and its connectives (Figs. 155, dlhc, 
193, 3 A). 

The Pharynx. — The pharyngeal part of the stomodaeum follows the 
buccal cavity (Fig. 190, PMj), and, if not structurally differentiated 
from the latter, it is to be identified as that part of the stomodaeum whose 
dorsal dilator muscles take their origin on the frontal and dorsal areas 
of the head wall and are inserted posterior to the frontal ganglion and 
its connectives (Fig. 155, Phy). The pharynx typically lies before the 
nerve connectives between the brain and the suboesophageal ganglion, 
but in some insects there is a second pharyngeal chamber of the stomo- 
daeum behind the connectives, the two parts being differentiated either 



Fig. 192. — Longitudinal vertical section of the head part of the stomodaeuni of a cock- 
roach, Blalta orientalis, showing precerebral anterior pharynx {APhy) and postcerebral 
posterior pharynx (JPPhy). (From Eidmann, 1924, but relellered.) 

by the contour of the stomodaeal tube or by their musculature or internal 
structure (Fig. 192). The precerebral and postcerebral pharyngeal 
regions are distinguished as the anterior pharynx (Figs. 192, 193, APhy) 
and posterior pharynx (PPhy), respectively. When there is no posterior 
pharjmgeal development the postcerebral region of the stomodaeum 
becomes a part of the oesophagus. In general the distribution of the 
dilator muscles serves better to identify corresponding morphological 
parts of the cephalic stomodaeum than does the structure of the parts 
themselves. The principal modifications of the pharynx and its muscula- 
ture have been sufficiently noted in connection with the feeding mech- 
anism described in the last chapter. 

The Oesophagus. — The oesophagus has no definite morphological 
status; it is merely the narrow part of the stomodaeum following the 
pharjmx that is not differentiated for purposes other than that of food 
conduction. T3T3ically the oesophagus is a slender tube and may extend 
direct to the stomach, but more generally it is limited posteriorly by a 
proventricular or ingluvial section of the stomodaeum (Figs. 190, 198, 199, 



THE ALIMEXTARY CANAL 


353 


Oc). When the inghn'ics, or crop, is a simple dilatation of the stomodaeal 
tube the oesophagus usually •widens gradually into the crop (Fig. 190), 
and the latter may extend so far forward as practically to exclude the 
oesophagus (Figs. 193, 195). 

The Crop, or Ingluvies. — The crop is ordinarily but an enlargement 
of the posterior part of the oe.sophagus (Figs. 190, 195, Cr). In some 
insect.s, however, it is a lateral diverticulum of the ocsojdiagus having 
the form cither of a simple sac (Fig. 197 B, C, Cr) or, as in some DijUera 
(Fig. 19S), of a long, slender tube with a bladderlikc .‘^welling at the end 
(Cr). The intima of the crop is usuallj' thick, the epithelium flat, and 
the walls of the entire organ, when not stretched bj* the food content, arc 
thrown into numerous lengthwise folds and transverse wrinkle.s that 
allow of distention. 

That the primarj’’ function of the crop is one of storage is ampl}’ 
attested by its size and structure. Most insects feed rapidlj’ when 
food is available in abundance and accomj)li.‘;h more leisurely the dige.s- 
tivc procc.sscs. Sanford (1918) found that cockroaches fed to repletion 
on a diet of oil and sugar could go for nearlj' two months before the croj) 
content was e.xhaustcd. It seems cquall}' clear, however, not merely 
that the crop is an antechamber of the ventriculus, or waiting room where 
the food is held in anticipation of its admission to the stomach, but that 
it is in itself the seat of a certain amount of food digestion, since it 
receives digestive liquids both from the .salivar}’ glands and from the 
ventriculus. Analyses have shown the presence of numerotis digestive 
cnz 5 'mcs in the stomodaeum of various insects, but all enzymes rejiorted 
from the stomodaeum occur also in the .salivary glands or the ventriculus 
or in both, and it is probable that these organs are the sources of the 
cnzj’mcs discovered in the croj>. A few writers have believed, however, 
that certain enzymes may be formed in the stomodaeal epithelium itself. 
Sanford (1918), for example, claimed that the fat-splitting enzyme lipase 
is a product of the croj) walls in the cockroach, and Swingle (1925) 
thought it likely that maltase and invert.ase .as well as lipase occurring 
in the crop must be produced there. On the other hand, both Abbott 
(192G) and IViggle.sworlh (1928) assert that lipase cannot be demon- 
.strated in the crop walls of the cockroach, and Abbott says the presence 
of lipase in the crop is the re.^jult of regurgitation from the stomach. The 
jiroduction of enzymes in the stomodaeum of any insect has, therefore, 
not yet been establi.'^hcd. 

The question .as to whether absorption takes place through the 
walls of the stomodaeiim is one al.'-'o that cannot bo regarded .as settled. 
Pctninkevitch (1900) and .Sanford (1918) have contended not only that 
absorption takes jdace in the crop, but that in the cockroach the crop 
is the chief seat of absorption. This claim they b.ase on liFtological 



354 


PRINCIPLES OF INSECT MORPHOLOGY 


studies of the crop epithelium of oil-fed roaches, the cells of which 
are found to be full of oil globules. Schliiter (1912), however, came 
to quite opposite conclusions from the same methods of study carried out 
on various orthopteroid species as well as on odonate larvae and on 
beetles. He asserts definitely that absorption does not take place in 
the crop, and that if fat appears in the ingluvial cells it gets there in 
some other way than by direct absorption from the crop lumen. Abbott 
(1926), again, agrees with Petrunke\dtch and Sanford that the crop of the 
roach is an important organ for the absorption of fat, but he says that 
water and water-soluble substances are not absorbed in it. 

The thickness of the stomodaeal intima would appear to be an effec- 
tive barrier to more than a minimum of absorption taking place in any 



Fla. 193. — The cephalic dilator muscles of the stomodaeum of a grasshopper, Dissostnra 

Carolina. 

part of the stomodaeum, and the experiments of Eidmann (1922) on the 
relative permeability of the intima of the crop and intestine in Blatta 
orientalis give little support to the idea of absorption in the former. 
Eidmann found that both alkaline and acid substances diffuse very 
slowly, only in the course of hours, through the intima of the crop, which 
has a thickness of 5 to 8 microns, though they penetrate the relatively 
thin intima of the intestine in 10 or 15 minutes. In other Orthoptera 
the crop intima is often much thicker than that of the cockroach, and, in 
general, it would appear that, as Schliiter remarks, “an organ could 
scarcely be made less fitted' for absorption.” 

In the Diptera the bladderlike crop (Fig. 198, Cr) usually contains 
a clear liquid. That of Tahanus, according to Gragg (1920), does not 
serve as a food reservoir; its contents are apparently derived from the 
mcsenteron and then again returned to the latter, a process that insures 
a thorough mixing of the gastric secretion with the ingested blood. 

The Proventriculus. — This, the terminal region of the stomodaeum, 
is often structurally the most highly specialized part of the alimentary 



THE All MEET AllY CANAL 


355 


cnnal. In its simpler forms, liowevcr, as seen in the larvae of man}’ 
insects and in some adults, it is merely the narrowed ])osterior end of 
the stomodacum which is more or less invapinated into the anterior end 
of the mesentcron to form the eardiae valve (I'ip. 139, .ST/r). 

In adult insects that feed on solid food the i)rov('ntricular repion 
usually becomes differentiated as a definite part of the alimentary tract 
between the crop and the ventriculus, aiul its inr.cr walls develop a 
mechanism, often armed with strong cuticular plates or teeth (Fip. 101), 
that may serve several purposes. The armature of the i)roventriculus 
lies anterior to the funnel-shaped posterior part of the orpan (iS'17c) 
. that enters the stomach, and for this reason it would appear to be a 
secondary addition to a more siinjde jjrimitive structure; but since some 
modification of the mechanism at least is present in most of the chewinp 
insects and also in the Hymenoptera, Eidmann (1921) suppests that the 
provcntricular armature is a primitive equipment of the insect alimentary 
canal, which has been lost in most of the sucking orders. 

The provcntricular mechanism con.'^ists fundamentally of strong 
longitudinal folds of the walls of the organ projecting into the lumen. 
The folds arc usually continuations of the le.'-s jnonounced plie.ations 
of the walls of the crop, and there are consequently four, si.\’, or eight 
major jn’ovcntricular folds and a varying number of juinor intermediate 
ridges. A simple condition is found in the Acrididae, where the walls 
of the inovcntriculus are produced into si.v longitudinal elevations 
(Fig. 191 B), each deeply grooved anteriorly and taj)ering postf'riorly 
to the margin of the short jiroventricular valve. Tin* surfaces of the 
lobes arc not strongly sclerotized in the grasstu!j)per, and they are arnn'd 
only with a few small marginal teeth and with areas of mitiute granul.a- 
tions on their distal halves. A lay(‘r of strong circular muscles runs 
continuou.sly around the i)roventriculus frnir/.v), but the longitudinal 
fibers are aggregated into si.\ proujis occui)ying the bases of the folds 
(Imcis). There appear to be also short inner transveise fibers in the 
crests of the folds (in) serving to compress the latter. By a contraelion 
of the circular muscles the si.\ major folds are evidently brought together 
and cfTcctively block the entrance to the ventriculus. Tin* ch.'innels 
between these folds, however, may permit the egress of ventricular 
litluids into the stomodacum, and the brown lirjuid that gr;i>‘-hoj)j>ens 
.sometimes eject from the nuaith jwobably esc;.j)es from the stomach 
in this manner. 

In the Blattidae fFip. 195) the si.\ mtijor folds of the provcntricular 
wall are densely sclerotired tinieriorly forming an arm.ature of six plates 
(n), each of which is produced centrally into a stronc, sharj) jirrw-ess 
with the point turned somewhat po.-teriorly. In tie* mon* la-ji'-ring 
posterior half of the pro\<-ntrieuIus behind the j)lat<^~ tie* fo!i!' are 



356 


PRINCIPLES OF INSECT MORPHOLOGY 


again thickened, forming here a circle of six soft, cushionlike lobes (6) 
covered with hairs or spines directed backward. The proventricular 
region is thus divided into a proveniriculus anterior armed with the 
plates, and a proveniriculus posterior containing the cushions. Beyond 
the cushions is the region of the stomodaeal valve {SVlv), which is a 
tn rh long, narrow tubular fold in the cockroach, 

Oe- 11 P on the inner walls of which the proventricular 

S ® folds are continued as low ridges that taper 

m M gradually to the end of the valve. A more 

M H detailed account of the pi oventriculus of the 

% cockroach is given by Sanford (1918) and by 

^ % Eidmann (1925). 

-^In c general structure of the proventriculus 

in the Gryllidae and Tettigoniidae is the same 
W as the Blattidae, but the sclerotic plates are 
|l 1 here longer and are broken up into series of 

a transverse ridges ending in points that appear 

Pvent GCa rows of overlapping teeth directed pos- 

Wf A teriorly. The proventriculus of Gryllus is 

described by DuPorte (1918), that of Gryllo- 
1 blattci by Sayce (1899), and that of Stenopel- 
by Davis (1927). 

M I M ^ The sucking insects usually lack a 

Vent: @ \ / 11 — 1 proventriculus, other than the region of the 

^ \ / \ cardiac valve, though the Siphonaptera are 

SVlv c said to have a proventricular region armed 

Fig. 194.— Section of the cuticular teeth. The flat, circular sac 

crop, proventriculus, stomodaeal . ' 

valve, and eardiac end of the that intervenes between the oesophagus and 

maullrier:taiL Stomach in muscoid Diptera, which most 

a, b, proventrieuiar plates and Writers refer to as the “ proventriculus ” (Fig. 

Cio'; 138, Car), is the anterior part, or cardia, of 

ventriculus; I, longitudinal the vcntriculus {Vent), as will later be 

shown. 

The function of the proventriculus unquestionably differs according 
to the structure of the organ and the nature of the food material in 


Pvent 


Vent 




' \ / 

\ ! 

SVlv 


Fig. 194 -Section of the ^.^^h cuticular teeth 
crop, proventriculus, stomodaeal 


muscles. 


different insects. In its simpler forms, as we have seen, it acts merely 
as a sphincter between the crop and the stomach to regulate the passage 
of food material into the latter. With the development of folds and 
sclerotic armature on its inner walls, however, the organ acquires a 
more diversified function. In the first place, the folds projecting into 
the lumen serve to hold back the food in the crop without completely 
closing the ventricular entrance. Digestive liquids from the stomach 
may thus be permitted to flow /orward into the crop through the channels 



THE ALI MENTAEY CANAL 


357 


between the folds and bring about a partial digestion of the crop food 
before the latter is transmitted to the stomach. This possible function 
of the proventriculus in Or(hoj)tcra and Coleoptcra has been particularly 
stressed bj-- Ramme (1913), who jmints out further that the movements 
of the proventricular lobes, brought about by the strong muscles sur- 
rounding the latter, must serve to mix the digestive fluids thoroughly 
into the food mass. Some insects accomplish an extraintcstinal digestion 
of the food, suj)i)osedl 3 ’ bj’ gastric juices ejected from the mouth. 

The armature of the proventriculus often has the form of convergent 
lamellae, and this t\-pc of structure has suggested that the apparatus 
serves as a strainer to jircvcnt larger pieces of hard indigestible matter 
in the food from entering the stomach, .such material being later disposed 
of bj" regurgitation, ^'he onlj* definite evidence of normal regurgitation 
bj' insects, however, pertains to Dytiscus, which is .said bj"^ Rungius (1911), 



Flo. 195. — TIic nlimpntnry cnniil, fi.alivnry clnmls, nnd Mnlpicliinn tubules of n ernsshopper, 

Distosictra Carolina. 

Ramme (1913), Blunck (191G), and others to disgorge indigestible parts of 
the animals on which it feeds. According to Blunck, Dytiscus has no 
salivary glands, and digestion takes place in the crop bj' liquids from the 
stomach. The provcntricidus, he .sa 3 's, grinds the food mass and pushes 
the larger fragments back into the crop, while at the same time it allows 
the liquefied residue to filter through into the stomach. A few hours 
after mealtime the beetle suddenl 3 ' ejects several times from its mouth a 
turbid cloud of material, which, as it disperses in the water, is seen to 
eontain undigested remnants of the food. Other insects, as far as 
observed, ordinarib’’ pass all undigested refuse through the stomach 
and intestine. Sanford (191S) observed regurgitation by overfed 
cockroaches, but it is here evidently the result of too much feeding and 
not an example of a normal ph 3 ’^siological process. 

The movements of the stomodaeum of Pcriplancta fuliginosa have 
been studied by Yeager (1931) who finds that peristalsis takes place 
in the crop in both a posterior and an anterior direction, and that the 
proventricular movements are contractile only. The acti\dties of the 
proventriculus, he says, appear to be largely controlled by the first 
thoracic ganglion of the ventral nerve cord. 



358 


PRINCIPLES OF INSECT MORPHOLOGY 


The earlier entomologists commonly regarded the proventriculus 
as a gizzard; judging from its structure in Orthoptera and Coleoptera 
they did not hesitate to name it the "chewing stomach” (Kaumagen). 
It was Plateau who first threw discredit on this idea, and later Ramme 
(1913) claimed to demonstrate that the proventriculus is in no case 
able to break up hard parts of the food. Much discussion has since 
ensued, and experimental e\ddence has seemed inconclusive. Recently, 
however, Eidmann (1924) has made observations that appear to be 
decisive. He finds that in cockroaches during moulting the post- 
cephalic part of the stomodaeal exuviae remains intact within the crop 
until the armature of the new proventricular cuticula is fully sclerotized; 
after this the old cuticula is broken up and the pieces discharged by 
way of the stomach and intestine. Furthermore, an examination of 



Fig. 19G. — The aliroentary canal, silk glands, dorsal blood vessel, and nerve cord of a 

caterpillar. 

the food content of the crop and ventriculus, made at a certain time 
subsequent to feeding, shows that the food particles of the latter are 
smaller than those in the crop. From these observations Eidmann 
concludes that the proventriculus anterior of the cockroach is a chewing 
apparatus, and that after trituration the food is returned to the crop 
where it undergoes a preliminary digestion by the enzymes of the salivary 
secretion. The food is then passed into the stomach through the proven- 
triculus posterior, which otherwise serves merely as a closing apparatus. 
Confirmatory evidence of the chewing function of the proventriculus 
is added by Da^is (1927), who inserted small strands of wax into the 
proventriculus of live Stenopelmatus and found the wax indented by the 
proventricular teeth. 

Finally, we may observe, the proventriculus serves in some cases 
as a stomach mouth {Magemmind), or pump (Pumpmagen, Emery, 1888). 
This function is particularly evident in the aculeate Hymenoptera. 
Here the four thick, inner lobes of the organ reach forward into the 
crop (honej’^ stomach of bees), and the posterior part e.xtends as a funnel- 
shaped tube into the ventriculus. The lobes open and close like a four- 
lipped mouth, and apparently it is by their activity that the food in -the 
crop is transferred to the stomach. 



THE ALIMENTARY CANAI. 


359 


The Cardiac Valve . — The cardiac, or stomodaeal, valve is essentially 
a circular fold of the stomodaeal wall projecting into the ventriculus 
from the posterior end of the stomodaeum (Fig. 194, SVlv). The valve 
is composed, therefore, of two cellular lamellae and is covered on each 
side by the stomodaeal intima. The basal ring of the outer lamella 
(d, d) marks the morphological terminus of the stomodaeum. In form 
the cardiac valve is generally cylindrical or funnel shaped, but it is not 
always symmetrically developed. The two lamellae are usually more 



Fig. 197. — Transformation of the alimentary canal of a moth, Malacosoma americana, from 
the larva (A) through the pupa (B) to the imago (C). 


or less free from each other and may include between them an extension 
of the stomodaeal muscles, but in some cases the two walls are adnate. 

The function of the cardiac valve is generally supposed to be that 
of preventing a return movement of the food from the stomach, but 
the fold does not entirely occlude the stomach entrance, since in some 
insects digestive juices flow forward from the latter into the crop. The 
projecting vahmlar tube conducts the food from the proventriculus 
well into the stomach lumen and partly shuts off a space around it in 
the cardiac end of the stomach, into which may open the gastric caeca, 
and in which may be situated special secretory cells of the ventricular 
Wall that form the peritrophic membrane (Fig. 204 A, B). 


4. THE MESENTERON 

The middle section of the alimentary canal (Fig. 190, Ment) is the 
stomach of the adult insect and is therefore commonly called the 
ventriculus. Only the epithelial wall of the ventriculus is formed from 



360 


PRINCIPLES OF INSECT MORPHOLOGY 


the endodermal mesenteron of the embryo (Fig. 13 D, Merit), but usually 
the entire adult organ is termed the mesenteron, or mid-gut. 

In the composite definitive alimentary canal the ventriculus begins 
morphologically at the base of the outer fold of the stomodaeal valve 
(Fig. 194, d), the line being marked by the termination of the stom- 
odaeal intima. The walls of the ventriculus are distinguished from 
those of the stomodaeum by the larger size and more spongy appearance 
of the epithelial cells, by the absence of a permanent or uniform intima, 
and by a reversal in the arrangement of the fibers in the muscular sheath. 



Fig. 198. — The alimentary canal and salivary glands of a fruit fly, Rhagolelis pomondla, 
ehoning the diverticular crop (Cr) and the cardiac sac (Cor) of the ventriculus, charac- 
teristic of many Diptera. 

the principal longitudinal muscles of the ventriculus (Fig. 201, ZmcZI 
being external to the circular muscles {cmcl). The ventriculus ends 
posteriorly a short distance before the bases of the Malpighian tubules 
(Fig. 190, Mai), which, when present, define approximately the anterior 
end of the proctodaeum. 

General Form of the Ventriculus. — The ventriculus commonly has 
the form of a tube or elongate sac of approximately uniform diameter 
(Figs. 195, 196, Vent). Only occasionally does it show a differentiation 
into regions, though in some insects it is quite distinctly divided into 
two, three, or four parts. 

The anterior end of the ventriculus surrounding the stomodaeal, or 
cardiac, valve is sometimes distinguished as the cardia (Figs. 190, 194 Car). 
In the muscoid Diptera the cardia becomes a small, flattened, circular 
sac containing the stomodaeal valve, separated from the rest of the 
ventriculus by a narrow constriction (Fig. 198, Car). Nearly all students 
of the alimentaiy canal of Diptera have called the cardia the “proven- 
triculus,” but its true nature is shown by the fact that the stomodaeal 
valve is invaginated into its anterior end (Fig. 204 B). In the mosquito 
(A) the cardia is less differentiated and is clearly the anterior part of the 
ventriculus. 



THE ALIMENTARY CANAL 


361 


In the horse fly Tahanus, as described by Cragg (1920), the ventriculus 
is differentiated into a slender anterior tubular region and into a posterior 
dilated region, the two differing both histologically and functionally as 
well as in form. The first part Cragg calls the “cardia,” though this 
term should be restricted to the anterior end of the ventriculus; the 
second he says is functionally the true stomach of the horse fly, since all 
the blood swallowed at the time of feeding is passed into it. 

The regional differentiation of the ventriculus is carried to its highest 
degree in the Hemiptera. In the more generalized Homoptera the organ 
is usually divided into three quite distinct parts (Fig. 209 A). The first 
part (1 Vent) is a sac lying within the filter chamber {FC ) ; the second is a 
large croplike enlargement (2 Vent ) ; and the third is a long slender tube 
(3 Vent), often called the "ascending intestine” since it turns forward to 



Fig. 199. — The alimentary canal of a scarabaeid larva, PopilUa japonica, with three seta 
of gastric caeca {IGCa, SOCa, SGCa). 

reenter the filter chamber. In the Heteroptera the ventriculus is com- 
monly differentiated into four well-defined regions differing in length 
and diameter (Fig. 200 B), the fourth being provided in many families 
with numerous caecal diverticula (GCa). The principal modifications 
in the form of the heteropteran alimentary canal are shown by Glasgow 
(1914) in a long series of figures. 

Caecal Diverticula of the Ventriculus, — Blind pouches varying in 
number and in length may be developed on different parts of the ven- 
triculus. Most commonly they occur at the anterior end surrounding 
the stomodaeal valve. There are usually from two to six of these anterior 
gastric caeca (Fig. 190, GCa), though the number may be greater. In 
form they are generally simple blunt or tapering processes, but in the 
Acrididae each is divided at its base into an anterior branch and a poste- 
rior branch (Fig. 195, GCa). Caecal diverticula sometimes occur, 
however, on other parts of the ventriculus, as in the larvae of lamellicorn 
beetles, in which there may be three circles of them (Fig. 199, IGCa, 2GCa, 
SGCa), two near the anterior end of the stomach, the other near the 
posterior end. In the larva of the fly Ptychoptera contaminata, van 
Gehuchten (1890) describes a circle of eight small diverticula near the 



362 


PRINCIPLES OF INSECT MORPHOLOGY 


anterior end of the ventriculus (Fig. 200 A, GCa), and a pair of long 
glandular pouches {gl) arising from the extreme posterior end of the 
organ. In manj^ Coleoptera a large part of the ventriculus is covered 
with small papilliform or sometimes elongate diverticula, but these struc- 
tures in most cases are the crypts of epithelial regenerative cells (Fig. 
206 C, Cpl) rather than true caeca. 

A remarkable development of caecal appendages on the ventriculus 
occurs in the Heteroptera, where in many families a group of diverticula, 



Fig. 200. — Examples of caecal diverticula on various parts of the ventriculus, and of 
subdi\-ision of the ventriculus. A, larva of Ptychoplera contaminata (Diptera) with 
glandular diverticula {gl) from posterior end of ventriculus. {From Van Gehuchten, 1890.) 
B. C, Pcliopetla abbreviata and Blissus leucoplerus (Heteroptera) with four sections in the 
ventriculus, and gastric caeca arising from the fourth section. {From Glasgow, 1914.) 

varjing greatly in number, size, and form, are given off from the fourth 
section of the stomach. An extensive study of the gastric caeca of the 
Heteroptera has been made by Glasgow (1914), who gives numerous 
illustrations of their various forms. In general there are two types of 
these organs: in one t 3 Tie the diverticula are short, of uniform size, and 
arranged in two or four rows along most of the extent of the fourth section 
of the stomach (Fig. 200 B, GCa)] in the other type the caeca are fewer 
in number but are long tubes of A'ai^dng length and often very unsym- 
metricallj’’ grouped (C). According to Glasgow the gastric caeca of the 
Heteroptera, wherever they occur, are invariably filled with bacteria, 
and the presence of the bacteria is hereditary, the organisms appearing 
early in the alimentary canal of the developing embryo. Glasgow says 
that “these normal bacteria appear not only to inhibit the development of 
foreign bacteria but to exclude them altogether.” He suggests, therefore. 



THE ALIMENTARY CANAL 


363 


that the function of the caeca is merely to provide a safe place for the 
multiplication of the normal bacteria of the alimentary canal. 

Histology of the Ventriculus The epithelial walls of the stomach 
are characteristically thicker than those of other parts of the alimentary 
canal, but the muscularis is usually more weakly developed than in the 
stomodaeal region. An intima is not always present, at least not in the 
form of a definite cuticular layer, and when it does occur as such it is 
continually or periodically shed into the lumen of the stomach. In 
most insects a thin peritropMc membrane surrounds the food contents of 
the ventriculus. 

The Epithelium. — The appearance of the ventricular epithelium 
(Fig. 201, Epth) varies greatly 
according to the state of the diges- 
tive processes. Most of its cells 
are columnar, with irregular inner 
ends more or less projecting into the 
stomach lumen. The cytoplasm 
appears granular or spongy; the 
nuclei are large and generally occu- 
py the middle or distal parts of the 
cell bodies, where, in sections, they 
form fairly even rows or hnes 
following the inner contour of the 
epithelium. In addition to these 
larger, spongy cells that form most 
of the epithelial wall, there are 
usually to be seen other smaller cells 
(rgf) of a denser texture occurring either singly or in groups between the 
bases of the larger cells (Fig. 202 B) or aggregated into definite 
clusters (C), sometimes contained in pockets, or crypts, of the 
epithelium (E). The larger cells, ha-idng their inner ends exposed or 
projecting into the stomach lumen, are the digestive cells (B, dg), that 
is, cells that take an active part in the processes of secretion or absorp- 
tion; the smaller basal cells are the regenerative cells (rg), the function 
of which is to propagate cells to replace the digestive cells when the 
latter are exhausted by secretory activities or shed at the time of ecdysis. 

The digestive cells constitute the functional epithelium of the stomach. 
Their central ends are often differentiated as a weakly staining, marginal 
layer of the epithelium, which, in sections, appears to be crossed by 
numerous fine lines perpendicular to the surface. This marginal zone 
of the stomach epithelium is known as the striated border (Figs. 201, 
202 A, sb). The nature of the striated border has been the subject of 
much discussion. Earlier investigators believed it to be a coating of 



Fig. 201. — Diagrammatic cross section 
of the ventriculus. BMh, basement mem- 
brane; cmd, circular muscles; Epth, epi- 
thelium; F, food material; IMd, longitudinal 
muscles; Lum, lumen; PMh, peritrophic 
membrane surrounding the food and sepa- 
rated by space (a) from epithelium; rg, 
regenerative cells; s6, striated border. 



361 


PRINCIPLES OF INSECT MORPHOLOGY 


fine filaments covering the inner surface of the stomach, comparable 
with the ciliate lining of the mesenteron in Annelida. In the insects, 
however, the striated zone is a continuous layer in which the darker 
fines of the striae alternate vdth clear fines of a less dense material 
through which minute droplets of digestive liquids may be extruded from 
the inner parts of the cells. The surface of the striated border is generally 
observed to be defined by a delicate limiting membrane. 

In most insects the digestive cells are of uniform structure throughout 
the ventriculus, except in that they may be of different sizes in different 



Fio. 202. — Diagrams shorving various positions of the regenerative cells (xo) of the ven- 
tricular epithelium with relation to the digestive cells (dg). 

parts of the stomach or may be found in various stages of disintegration. 
In the larvae of Lepidoptera, howcAmr, there are two quite distinct types 
of digestive cells. Those of one type have the ordinary columnar or 
cylindrical form; those of the other, characterized as calyciform or goblet 
cells, have each a large ampulla in its mesal part opening by a narrow 
neck through a small aperture on the inner surface. The two types of 
digestive cells of the caterpillar have been studied particularly in Galleria 
mcUonclla bj’- Yung-Tai (1929), who finds that they are differentiated 
oven in the embryo, and that they are generated separately from the 
replacement cells. The cavities of the goblet cells are fined Avith a 
striated border like that of the columnar cells. Yung-Tai concludes 
that the goblet cells are exclusiA'ely secretory in function, Avhile the 
ordinary cylindrical cells may be either secretory or absorptiA'^e, though 
the same indiA’idual cells do not function in both capacities. The goblet 
cells, he says, are not replaced after the moult to the pupa. 

From its A-ery beginning the endoderm of insects appears to be an 
unstable tissue. As Ave saAV in Chap. II, the formation of the mesenteron 
in the embrjm is apparently a regeneratiA^e process following an earlier 



THE ALIMENTARY CANAL 


366 


dissolution of the primitive archenteron. During postembryonic life of 
most insects the cells of the mesenteric epithelium are continually sub- 
ject to various degrees of disintegration as a result of their secretory 
processes, or for the purpose of reconstructive growth in the ventriculus 
accompanjdng the moults. The replacement of the epithelium is either 
gradual and partial or rapid and complete, according to the nature of 
the disintegration processes. The new cells are formed from the special 
regenerative cells, which take no part in the other activities of the 
ventriculus. The processes of regeneration are in general the same 
regardless of the degree, time, or manner of cell replacement or the 
reason for its occurrence. 

Both the digestive and the regenerative cells of the ventriculus are 
derived from the primitive endoderm, the digestive cells being so special- 



Fio. 203. — Sections of the ventriculus of a mosquito larva and pupa shouang regener- 
ative cells. (From Samtlcbcn, 1929.) A, B, Culcx pipiens, middle-aged larva. C, 
Aides mcigcnamis, newly forming pupa. D, Culcx pipiens, larva just before pupation. 

ized for the functions of secretion and absorption that they have lost the 
power of reproduction, while the regenerative cells maintain unimpaired 
the property of mitotic division. The regenerative cells are usually of 
small size and lie beneath the others against the basement membrane 
(Fig. 202 B, rg). They are shown in a relatively simple condition in 
Collembola of all stages and in some dipterous larvae (Fig. 203, rg), where 
they occur singly or in small groups scattered throughout the length of 
the ventriculus. 

The regeneration cells of the ventricular epithelium generally form 
definite cell groups, or nidi, sharply distinguished from the surrounding 
digestive cells (Figs. 202 C, 205 B, 206 A, rg). From these specialized 
regeneration centers are propagated the new cells that replace the 
exhausted or discarded digestive cells. In the Hymenoptera the regener- 
ation cells are contained in open pockets of the ventricular epithelium, 
but in other insects in which the regeneration cells are grouped in definite 
nidi, the pockets are generally closed by an overgrowth of the surrounding 
digestive cells (Figs. 202 C, 206 A), and the general contour of the inner 
surface of the epithelium gives no indication of the position of the nidi. 
The regeneration cells, however, may lie at the bottoms of deep folds or 



366 


PRINCIPLES OF INSECT MORPHOLOGY 


pockets of the epithelium. In some Coleoptera they are contained in 
evaginations of the stomach wall, forming pouchlike diverticula known 
as the regenerative crypts (Figs. 202 E, 206 C, Cpt), which may be so 
numerous as to give the entire external surface of the ventriculus a villous 
structure. 

The Basement Membrane. — ^The epithelial cells of the ventriculus rest 
upon a membrane (Fig. 201, BMh) w'hich appears to be a tunica propria, 
or product of the cell bases, differing in no respect from the basement 
membrane of the body wall or from that of the ectodermal parts of the 
alimentary canal. According to Deegener (1910) and Rungius (1911; 
Korschelt, 1924), however, the ventricular epithelium of Dytiscus is 
invested in a thick supporting layer {Sliitzlamelle) which is a nucleated 
connective tissue and is not to be identified with the tunica propria of the 
stomodaeum and proctodaeum. 

The PeritropMc Membrane. — ^The food content of the stomach in 
many insects is separated from the ventricular epithelium by a thin 
membrane, which, though often in more or less intimate contact with the 
inner ends of the epithelial cells, typically surrounds the food mass as a 
cylindrical sheath for the most part free from the stomach walls. This 
food envelope is known as the peritrophic membrane (Fig. 201, PMh). 
It is not present in all insects, but it is known to occur in Collembola, 
Thysanura, Ephemerida, Odonata, Orthoptera, Neuroptera, Coleoptera, 
Hymenoptera, Diptera, and larval Lepidoptera, while it is said to be 
absent in Hemiptera and adult Lepidoptera, as well as in certain members 
of the orders in which it is usually present. 

The peritrophic membrane is a product of the ventricular epithelium, 
being formed in most cases from the entire surface of the ventriculus, 
but in Diptera it appears to be produced by a band of specialized cells in 
the anterior end of the cardia encircling the base of the stomodaeal valve. 
In no case is it a continuation of theproventricularintima. Somewriters 
have assumed that the peritrophic membrane is a nonchitinous structure 
because it is produced by endodermal cells; but several investigators, 
including Wester (1910), Campbell (1929), and von Dehn (1933), have 
found by chemical tests that the peritrophic membrane contains chitin, 
while Hovener (1930) says that it shows two characteristic properties of 
chitin, namely, double refraction and resistance to alkalies. There is no 
reason for supposing that chitin should not be produced from endodermal 
as well as from ectodermal derivatives of the blastoderm; the peritrophic 
membrane is e\’idently to be regarded as a chitinous intima of the 
ventriculus. 

The component material of the peritrophic membrane is probably 
a secretion product of the matrix cells. Folsom and Welles (1906) 
claimed that the peritrophic membrane of Collembola is a direct trans- 



THE ALIMENTARY CANAL 


367 


formation of the striated border of the ventricular epithelium, cast off 
from time to time, as a new striated border is formed beneath it, and 
Ertogroul (1929) described the peritrophic membrane in the silkworm 
as formed in the same manner. According to Yung-Tai (1929), however, 
the peritrophic membrane of the larva of Galleria mellonella consists of 
successive delaminations of a surface membrane of the ventricular epi- 
thelium, and that of the larva of Fanessa urticae is said by Henson (1931) 
to be a secretion product of the epithelial cells. Von Dehn (1933) 
contends that the peritrophic membrane is in no case identical with the 
striated border of the epithelium, since the membrane is chitinous and 
the striated border is C 3 doplasmic. The chitinous material of the peri- 
trophic membrane, she says, appears in liquid droplets beneath the 
striated border, is extruded through the interstices of the latter, and 
runs together over the cell surfaces to form a continuous layer, which is 
then separated from the epithelium to become a peritrophic membrane. 
In the process of separation the striated border may be more or less dis- 
rupted and fragments may adhere to the membrane, but not as constit 
uent parts of it. 

In the larvae of most aculeate Hymenoptera the peritrophic mem- 
branes form a sac closed posteriorly about the food mass of the stomach, 
since the mesenteron does not open into the proctodaeum until the 
termination of larval life. The same is said to be true of the larvae of 
some Neuroptera. The food sac of a mature wasp larva {Vespa) appears 
as a bag filled with a black mass, the bag lying free in the ventriculus 
except for an attachment to the walls of the latter around the base of the 
stomodaeal valve. At the time of defecation the sac becomes detached 
and is ejected entire ■with its contents into the inner end of the cocoon, 
which the larva has already spun about itself, and here the dejectamenta 
dry to a hard black mass. The peritrophic membranes of the wasp 
larva are described by Rengel (1903), and those of ant larvae by Strmd- 
berg (1913), as being given off successively from the general surface of 
the ventricular epithelium. In the larva of the honey bee Nelson (1924) 
describes the peritrophic membrane as a thick homogeneous layer, appar- 
ently of gelatinous consistency, covering the inner surface of the epithe- 
lium, but at the anterior end of the ventriculus he says there is a ring of 
specialized cells from the surface of which streams of secretion issue and 
run caudad to join with the principal mass of the peritrophic membrane. 

Since the peritrophic sac of hymenopterous larvae becomes enthely 
free from the walls of the ventriculus in the mature larva, except for its 
anterior attachment, it is clear that an examination of a larva at this 
stage would suggest that the membrane is a product only of the ring of 
specialized cells, noted by Nelson in the honeybee, surrounding the stomo- 
daeal valve. The statement by Cuenot (1896) that the peritrophic 



368 


PRINCIPLES OF INSECT MORPHOLOGY 


membrane of Orthoptera is the product of secretion, by cells occupying 
the anterior end of the mesenteron must, therefore, be taken with some 
reserve, especially since Davis (1927) finds the food envelopes of Sieno- 
pchnahts to be mostly a series of delaminations from the entire surface 
of the stomach epithelium, though possibly augmented from the secretion 
of special anterior cells. On the other hand, there appears to be reason 
to believe that in the Diptera, both larvae and adults, the peritrophic 
membrane may take its origin entirely from a band of specialized cells 
confined to the anterior end of the A'^entriculus. 

In the adult honey bee the food material of the ventriculus is usually 
enclosed in a series of peritrophic membranes, which are given off suc- 
cessively from the inner surface of the epithelium. During periods of 
secretory activity the secretion products formed in the cells accumulate 
beneath a surface film, or border membrane, and the whole mass eventu- 
ally separates from the cell layer, which then forms a new border mem- 
brane. Most of the discarded substances are dissolved, and the residue 
becomes a peritrophic membrane. 

That the peritrophic membrane of Diptera is produced from cells in 
the anterior end of the mesenteron was first suggested by van Gehuchten 
(1890) ; but van Gehuchten called the cardiac enlargement of the mesen- 
teron (Fig. 198, Car) the “pro ventriculus," and this terminology, adopted 
by many subsequent students of the alimentary canal of Diptera, has 
been a source of confusion to those who have not perceived that the organ 
in question is not the proventriculus of other insects (Fig. 190, Pvent) 
but is the cardiac section of the mesenteron. The cardia is best developed 
as an antechamber of the stomach in the muscoid Diptera, where it takes 
the form of a flattened, circular sac (Fig. 198, Car) with the stomodaeal 
valve invaginated into its anterior end (Fig. 204 B, SVlv). In the lower 
flies this region of the stomach is less differentiated, but it is recognized 
by Imms (1907) in the mosquito larva as the cardia (Fig. 204 A, Car). 
The anterior part of the wall of the cardia is formed by a band of special- 
ized epithelial cells (e) surrounding the base of the stomodaeal valve 
(SVlv), and it is these cells apparently that secrete the substance which 
forms the peritrophic membrane {PMb). 

The formation of the peritrophic membrane in Diptera as an apparent 
secretion from a ring of specialized cells in the anterior end of the mesen- 
teron has been described by Haseman (1910) and Hovener (1930) in 
Psychoda allcrnata, and by Wigglcsworth (1930) in Glossina. In Psy- 
choda, according to Haseman, the glandular membrane-forming cells 
occupj’^ a circular area 6 to 12 cells in length just beyond the base of the 
stomodaeal valve; the inner granular surface of the cells hardens to form 
the delicate peritrophic membrane, “which is continually fed back into 
the mid- and liind-intcstine to envelop the food materials.” In the 



THE ALIMENTARY CANAL 


369 


description of the formation of the peritrophic membrane of the tsetse 
fly given by Wigglesworth the reader must understand that the term 
‘'proventriculus” refers to the cardia, or anterior end of the mesenteron. 
Figure 204 C, based on Wigglesworth’s drawings of Glossina, shows a 
section through one side of the stomodaeal valve (SVlv) and the wall of 
the surrounding cardia (Car). As in most of the higher Diptera the lips 
of the valve are reflected forward, and in the circular space thus enclosed 
at the base of the valve is the ring of large secretory cells (e) of the 
anterior end of the mesenteric epithelium. The discharged products of 
these cells condense to form a cylindrical peritrophic membrane {PMh) 
fed back into the ventriculus from around the periphery of the reflected 
lips of the stomodaeal valve. In the mosquito larva, as shown by TmmR 



Flo. 204. — Sections of the stomodaeal valve, cardia, and peritrophic membrane of 
Diptera. A, Anopheles maculipcnnis larva. (From Imms, 1907.) B, Calliphora ery- 

throeephala, stage almost adult. (From Ptrez, 1910.) C, Glossina adult, one side of 
stomodaeal valve and wall of cardia. (Diagrammatic from Wigglesworth, 1929.) 


(1907), the peritrophic membrane is formed in a similar manner (Fig. 
204 A), but the generative cells here occupy most of the length of the 
cardia. 

Inasmuch as the peritrophic membrane, when present, usually com- 
pletely surrounds the food content of the stomach (Fig. 201), the products 
both of epithelial secretion and of gastric digestion must penetrate the 
membranous envelope, the first to act upon the food, the second to be 
absorbed by the ventricular cells. The space between the epithelium 
and the peritrophic membrane (a) is generally filled with digestive liquid, 
granules, globules of secretion products, discharged epithelial cells, and 
presumably also with food material in solution that has passed outward 
through the peritrophic membrane. The permeability of the peritrophic 
membranes of the honey bee and the blow fly to various stains has been 
demonstrated by von Dehn (1933). At present no satisfactory explana- 
tion can be offered as to the general function of the membrane, which 
occurs also in other arthropods than insects. 



370 


PRINCIPLES OF INSECT MORPHOLOGY 


The Muscularis.—The muscular sheath of the ventriculus is less 
strongly developed than that of the stomodaeum. The circular fibers 
(Fig. 201, cmcl) generally constitute the principal layer, the longitudinal 
fibers {Imcl), bung external to the circulars, being usually widely spaced, 
and sometimes groups of longitudinal fibers form special lengthwise 
muscle bands that look like cords stretched between the two ends of the 
stomach. Muscles of the latter type are particularly conspicuous in the 
caterpillars (Fig. 207 A, B, Vent). "V^Tiile in most insects all the longitu- 
dinal muscles of the ventriculus lie external to the circular muscles, a few 
lengthwise fibers are said by Rengel (1898) to lie within the circular 
fibers in Hydrophilus, and White (1918) claims that there is likewise in 
the honey bee an inner layer of very fine longitudinal fibers between the 
circular muscles and the basement membrane of the ventricular epithe- 
lium. A peritoneal covering of loose cellular tissue is said by some 
writers to surround the muscularis in certain insects, but usually the 
muscles of the alimentary canal have no very definite investiture. 

Activities of the Ventricular Epithelium. — The activities of the 
epithelial cells of the ventriculus may be divided for descriptive purposes 
into four classes, as follows; (1) secretion and absorption, (2) excretion, 
(3) degeneration and regeneration of the digestive cells accompanying 
or following secretion, and (4) periodical delamination and replacement 
of the entire epithelium, mostly accompanying the moults. 

Secretion and Absorption. — The primary functions of the cells of the 
ventricular epithelium are the production of liquids containing digestive 
enzjones, and the absorption and transmission to the blood of the products 
of digestion. Probably in most insects both these activities are prop- 
erties of the same cells, but van Gehuchten (1890) has claimed that the 
two functions pertain to two sets of cells in the fly Ptychoptera contami- 
nata, and Yung-Tai (1929) gives convincing evidence that the goblet 
cells in the larval epithelium of the moth Galleria mellonella are exclu- 
sively secretory, while the columnar cells may be either secretory or 
absorptive in function, though the two activities are not performed by 
the same cells of this group. 

The discharge of the secretion products, in its simplest form, 
undoubtedly, is accomplished by the direct passage of the elaborated 
substances through the striated border of the secreting cells, and it is 
possible that the secretion discharge in all cases takes place by this 
method. With most insects, however, there is to be observed in the 
ventriculus a conspicuous process of budding from the inner ends of the 
epithelial cells. The extruded globules either disrupt and scatter their 
contents in the ventricular lumen or they become detached and are given 
off as free bodies which later disintegrate. Generally it has been sup- 
posed that these acti\ities of the ventricular cells, which have been 



THE ALIMENTARY CANAL 


371 


studied only as physical phenomena in histological preparations, are 
processes of holocrine secretion, but there is a recent tendency to regard 
them as disintegration processes following exhaustive periods of ordinary 
secretion. In any case they are anatomically degenerative changes and 
will be described in this category. 

Excretion . — ^There is little doubt that the walls of the ventriculus 
play some part in excretion, in either an active or a passive role. The 
epithelial cells are often observed to contain large numbers of small 
crystalline bodies, which are found to be principally calcium salts, though 
some also are said to have the properties of uric acid concretions. Such 
deposits, together with bacterial inclusions, are at least eliminated with 
the shedding of the epitheUvun at the time of ecdysis. 



Fig. 205. — Examples of disintegration processes (supposedly holocrine secretion) in the 
stomach epithelium. A, Ptychoptera contaminata larva. {From Van Gehuchten, 1890.) 
B, Gomphus descriptua larva. {From Needham, 1897.) C, Tabanua adult. {From Gragg, 
1920.) 


Degeneration and Regeneration of the Digestive Cells . — Throughout 
the active life of most insects there takes place in the epithelium of the 
ventriculus a partial or complete disintegration of the digestive cells, 
followed by a replacement of the lost cells with new cells formed from the 
regeneration cells. 

The simplest form of disintegration in the digestive cells consists of 
the accumulation of granular material in the inner ends of the cells, 
succeeded by a rupture of the cell wall and the discharge of the material 
into the ventricular lumen. The cell wall then closes, the striated border 
is reestablished, and the cell continues its digestive functions. 

A second and more intensive form of disintegration involves a separa- 
tion of the inner parts of the cells containing the granules and globules. 
The mesal border of the cell in this case swells out in the form of a bud, 
which becomes constricted at its base and finally separated as a free 
sphere from the body of the cell (Fig. 205 A). In Collembola, according 
to Folsom and Welles (1906), the bud is at first surrounded by a striated 
zone, which later is lost; but in most other insects the striated border 
disappears on the evaginating bud. The liberated sphere floats off into 



372 


PRINCIPLES OF INSECT MORPHOLOGY 


the stomach lumen and there undergoes a gradual dissolution which sets 
free its contents. This form of disintegration is the one generally 
observed in adult insects. The buds vary from rounded protuberances 
(A, h) to fingerlike processes or appear as small globules at the ends of 
long slender stalks (C, h). In most insects the buds are formed prior to 
feeding, and, as shown by Needham (1897) in odonate larvae, they may 
in such cases increase enormously in size and numbers in starved indi- 
^'iduals. In the horse fly Tabanus, however, described by Cragg (1920), 
the buds are extended during the period of feeding, and after their dis- 
charge the epithelial cells go back at once to the normal resting condition. 
The horse fly, Cragg says, feeds at intervals of two or three days. 

A third type of cell disintegration is similar to the last, except that 
the part of the cell given off contains a nucleus and is, therefore, an 
extruded cell. The cell, loaded with granular matter, degenerates and is 
dissolved in the stomach lumen. The liberation of nucleated cells. 
Needham says, is characteristic of dragonfly larvae (Fig. 205 B) ; in other 
insects it frequently accompanies the discharge of nonnucleated bodies, 
as in the horse fly (C, c) and in the honey bee. This form of disintegration, 
as well as the last or the two together, results in a rapid and extensive 
depletion of the digestive cells of the epithelium, necessitating their 
replacement by cells propagated from the regenerative cells. 

All these forms of cell disintegration in the ventricular epithelium 
have generally been described as methods for the rapid discharge of 
secretion products. Only recently this interpretation is challenged by 
Yung-Tai (1929), who points out that secretions are always in the form 
of a diffusible liquid, and that the coarse granular contents of the buds 
and globules given off from the digestive cells have all the aspects of 
cytoplasmic degeneration products. He therefore contends that the 
processes ordinarily described as secretion discharge are really disintegra- 
tion processes follovdng active periods of secretion or absorption. This 
view is endorsed also by Henson (1930). The subject, however, must 
be studied from a physiological standpoint before conclusions can be 
justified. 

Periodic Delaminalion and Replacement of the Ventricular Epithelium . — 
Reconstructive processes, varying in degree, usually occur in the stomach 
walls at the time of ecdysis, particularly at the moult of the larva to the 
pupa in holometabolous insects. In some insects the entire ventricular 
epithelium is shed and renewed at each moult, and in certain beetles a 
complete regeneration of the stomach wall is said to occur periodically 
throughout adult life. 

A replacement of the entire ventricular epithelium accompanjdng 
each ecdysis has been described by Folsom and Welles (1906) and by 
Boelitz (1933) in Collembola, which moult throughout life, and a similar 



THE ALIMENTARY CANAL 


373 


process accompanying the larval ecdyses has been observed in Dermes- 
tidae by Mobusz (1897) and Braun (1912), in the moth Galleria melonella 
by Yung-Tai (1929), and in the fly Psychoda alternata by Haseman 
(1910). The renewal of the ventricular epithelium in Collembola, 
according to Boehtz, is preceded by an evacuation of the stomach and 
starts with mitotic division in the regenerative cells, the activity of the 
latter beginning anteriorly and proceeding posteriorly. As the new cells 
multiply, the old epithelium is separated from the basement membrane, 



Fig. 206. — Regenerative cells of the ventricular epithelium. A, a nidus of regener- 
ative cells of Stenopelmalus. {From Davis, 1927.) B, a crypt of regenerative cells of larva 
of Dytiscus marginalis. {From Rungius, 1911.) C, two crypts of adult Hydrophilus 
piceus. {From Bengel, 1898.) D, same during regeneration of new epithelium {SEpth), 
with old epithelium {lEpth) cast off. 


which remains intact, and is pushed toward the lumen of the stomach, 
finally to be thrown off into the latter, where it is digested and absorbed 
by the new epithelium. Folsom and Welles described the rejected 
epithelium of Collembola as formed by a longitudinal division of the 
primary epithelium, the outer layer remaining as the next functional 
epithelium; their account makes no mention of the regenerative cells 
later described by Boelitz. 

In most holometabolous insects there is probably more or less of a 
renovation of the stomach epithelium accompanying each moult of the 
larva; but in the majority of cases observed the renovation does not 
involve a complete loss of the old cell wall. According to Braun (1912), 
in species of Lepidoptera, Coleoptera (except Dermestidae), Hymenop- 


374 


PRINCIPLES OF INSECT MORPHOLOGY 


tera, and Diptera studied by him, active cell division and epithelial 
growth, in some cases accompanied by the loss of a few cells thrust out 
into the stomach lumen, take place during the periods of larval ecdysis. 
These activities of the mesenteron cells, however, he says are primarily 
for the purpose of growth in the alimentary canal following the moult, 
and only to a small degree do they have a regenerative significance. 

At the penultimate moult of holometabolous insects, that is, with 
the change from the larva to the pupa, it is well known that the ventricular 
epithelium is cast off and replaced by a new cell layer that takes on more 
nearly the form of the ventriculus of the adult insect. Most investiga- 
tors find that the pupal, or imaginal, epithelium is formed from the same 
regenerative centers that produce the new cells of the larval ventriculus. 
The statement by Mansour (1928), therefore, that the imaginal stomach 
of Rhynchophora is generated from the stomodaeum and not from the 
cells of the larval mesenteron, if true, would establish a most exceptional 
condition in these beetles, since it implies, as Mansour claims, that the 
imaginal stomach is of ectodermal origin. 

A replacement of the stomach epithelium between the pupal and 
imaginal stages has not been generally observed, but Deegener (1904) 
says that the epithelium is renewed at the pupal moult in the beetle 
Cyhislcr, and Russ (1907) describes a partial degeneration and replace- 
ment of the pupal epithelium in Trichoptera. 

Finally, it appears that a complete renewal of the stomach epithelium 
may occur even in the imaginal instar of pterygote insects. Rengel 
(1898), for example, claims that a periodic shedding and regeneration 
of the entire ventricular epithelium take place in members of the Hydro- 
philidae throughout the lifetime of the adult beetles. He describes 
both processes in detail for Hydrophilus (Fig. 206 D). Though he says 
nothing of the physiological significance, it is to be supposed that the 
shedding of the old cell layer (lEpth) is a preliminary to the renewal 
of the epithelium (2EptJi) following exhaustion from secretory activities. 
The old epithelium is entirely replaced by a new cell layer (C, Epth) 
formed from the regenerative cells (rg) of the ventricular crypts (Cpt). 

5. THE PROCTODAEUM 

The proctodaeum is the posterior ectodermal part of the alimentary 
canal. In its lesser degrees of development it is a simple tube (Fig. 189, 
Proc) constituting merely a conduit from the stomach to the anus; but, 
as the stomodaeum, the proctodaeum also is generally differentiated into 
several more or less distinct regions. The anterior end of the procto- 
daeum is approximatel}'’ marked by the bases of the Malpighian tubules 
(Fig. 190, Mai), since these tubules are diverticula of the proctodaeal 
walls; but tlie true dividing line between mesenteron and proctodaeum 



THE ALIMENTARY CANAL 


375 


is usually somewhat anterior to the bases of the tubules, and in some 
insects it lies a considerable distance before them. The entrance to the 
intestine from the stomach is generally more or less constricted, and 
the opening is guarded by a regulatory structure commonly known as the 
'pyloric valve (Fig. 189, PVlv). The analogy with vertebrate anatomy 
implied in the term, however, is not exact, for the valvular apparatus 
in insects is usually, though not always, located behind the stomach in 
the anterior part of the proctodaeum. 

The proctodaeum is furnished with extrinsic muscles that extend 
to its posterior parts from the wall of the abdomen. These muscles, 
often called the “suspensory” muscles of the proctodaeum, probably 
serve in part to maintain the position of the intestine, but they evidently 
have also a more active function. In some insects they are clearly 
dilators of the proctodaeum, since they spread in fan-shaped bundles 
from their origins to their insertions on the proctodaeal walls ; in others, 
as in the caterpillars (Fig. 207 A), where they take a more longitudinal 
course, it would appear that they play some part in evacuation. 

Subdivisions of the Proctodaeum. — ^The regions into which the procto- 
daeum is usually differentiated vary in different insects, and for this 
reason it is difficult to apply a consistent terminology to them. The 
names by which they are conunonly designated are borrowed from human 
anatomy, and they have no excuse in entomology other than that of 
nomenclatural convenience. 

The most general division of the proctodaeum is into an anterior 
intestine (Fig. 190, AInt) and a posterior intestine {Pint), the second 
being commonly termed the rectum {Reel). The two parts are usually 
separated externally by a sharp constriction, and internally by a rectal 
valve. In many insects, however, there is a short but distinct section 
of the proctodaeum that intervenes between the ventriculus and the true 
intestinal tube, which contains the sphincter valve that regulates the exit 
from the stomach. This section is the pylorus {Py). The Malpighian 
tubules {Mai) open into the anterior part of the proctodaeum, sometimes 
immediately behind the ventriculus; but when there is present a distinct 
pyloric region, they discharge into the latter. 

The anterior intestine may be a simple tube, varying in length in 
different insects, but it is often subdivided into an anterior ileum (Fig. 190, 
II) and a posterior colon {Cln). The posterior intestine is generally 
dilated anteriorly into a rectal sac {rsc) and narrowed posteriorly in a 
straight tubular part, or rectum proper {rect), that goes direct to the anus 
{An) . Frequently the anterior intestine opens into the posterior intestine 
on the side of the rectal sac (Fig. 210 B), and in such cases the anterior 
end of the latter becomes a blind pouch, or rectal caecum. In some of the 
Heteroptera almost the entire proctodaeum consists of a large sac 



S76 


PRINCIPLES OF INSECT MORPHOLOGY 


(Figs. 200 B, C, Red, 219 A, r). If this sac is the rectum, as it appears 
to be, a short tubular invagination of the intestinal wall behind the 
swollen bases of the Malpighian tubules is perhaps a remnant of the 
anterior intestine. 

Histology of the Proctodaeum. — ^The walls of the proctodaeum 
resemble in structure those of the stomodaeum. The cells of the epithe- 
lium are flat or columnar, in most places showing little evidence of having 
a secretory function, and they are covered internally with a distinct 
cuticular intima. The muscle layer of the proctodaeum is less regular 
than that of the other sections of the alimentary canal and is frequently 
absent on some of the intestinal regions. In general the muscularis 
includes internal circular fibers and external longitudinal fibers, resem- 
bling thus the muscle sheath of the ventriculus rather than that of the 
stomodaeum; but the relative development of the two sets of fibers often 
varies greatly in different parts of the proctodaeum, and there may be 
additional muscles either outside or inside the usual layers. Special 
histological features of the proctodaeum will be described in treating 
of the several intestinal regions individually. 

The Pylorus. — The anterior part of the proctodaeum is often differ- 
entiated as a well-defined region into which open the Malpighian tubules 
(Fig. 207 A, B, Py). Since the pyloric valve is usually situated here, this 
region is termed the pylorus (“gatekeeper”) of the intestine (Deegener, 
1904; Rungius, 1911, 1924; Weber, 1933), though the term in vertebrate 
anatomy applies to the posterior part of the stomach. Examples of a 
well-differentiated pyloric region are to be found in Coleoptera and in 
the larvae of Lepidoptera. In some insects, however, there is no pyloric 
valve other than a small epithelial fold between the mesenteron and 
proctodaeum, and in such cases there is consequently no differentiation 
in the external structure of the alimentary tube to distinguish a pyloric 
region from the rest of the intestine. 

In the caterpillars the pylorus constitutes a distinct and highly 
specialized proctodaeal region (Figs. 196, 207 A, Py) between the ven- 
triculus (Feni) and the enlarged middle chamber (Alnt) of the intestine. 
The Malpighian tubules (Mai) open into its posterior part. The organ, 
when fully stretched out (Fig. 207 A, Py), presents a narrow posterior 
neck or stalk surrounded by a strong, external sphincter muscle (spir) 
just behind the bases of the Malpighian tubules (Mai), and a widened 
anterior part continued forvmrd as a calyxlike expansion continuous 
with the posterior end of the ventriculus (Vent). The line between the 
mesenteron and the pylorus is marked externally by a strong band of 
circular muscles (A, g), and internally by a corresponding fold (B, g). 
The proctodaeal intima up to this fold is covered with small spicules. 
Midway in the walls of the anterior part of the pylorus there is a second 



THE ALIMENTARY CANAL 


377 


internal fold (A, B, h) which varies in height according to the contraction 
of the organ. The entire length of the pylorus is closely surrounded 
by a series of circular muscle fibers, outside of which there are widely 
spaced, branching longitudinal muscles (A, Imcl) that are free from the 
pyloric walls except at their ends. Posteriorly these muscles pass beneath 
the sphincter {sptr). In appearance the pylorus of the caterpillar varies 
much according to the state of contraction of the longitudinal muscles; 



Fig. 207. — The prootodaeum, pylorus, and pyloric valve. A, proctodaeum of a 
noctuid caterpillar, showing highly developed pylorus {Py). B, internal view of pylorus 
of same in a contracted condition. C, section of proctodaeal pyloric valve iPVlv) of adult 
Phyllophaga gracilis. {From Fletcher, 1930.) g, junction of mesenteron and proctodaeum; 
h, fold of pyloric wall. 

in the same species it may be stretched out, as in Fig. 207 A, or again it 
may be contracted and thrown into strong circular folds as at B. 

The Pyloric Valve .- — Two different types of valvular structures are 
associated with the opening from the stomach into the intestine. In 
some insects a small, internal, circular fold, or ring of long cells, projects 
from the posterior margin of the mesenteric epithelium, forming a 
ventricular valve; in others an apparatus for closing the entrance into the 
intestine is developed in the pyloric region of the anterior end of the 
proctodaeum and constitutes a proctodaeal valve. The latter, when 
present, is clearly the more efficient occlusor mechanism and is the one 
generally found at t